[RFC 21/24] backup: move to block-copy

[Vladimir Sementsov-Ogievskiy]

This brings async request handling and block-status driven chunk sizes
to backup out of the box, which improves backup performance.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsement...@virtuozzo.com>
---
 qapi/block-core.json       |   9 +-
 include/block/block_int.h  |   7 +
 block/backup.c             | 159 +++++++++++--------
 block/replication.c        |   1 +
 blockdev.c                 |   5 +
 tests/qemu-iotests/129     |   3 +-
 tests/qemu-iotests/185     |   3 +-
 tests/qemu-iotests/219     |   1 +
 tests/qemu-iotests/257     |   1 +
 tests/qemu-iotests/257.out | 306 ++++++++++++++++++-------------------
 10 files changed, 276 insertions(+), 219 deletions(-)

diff --git a/qapi/block-core.json b/qapi/block-core.json
index aa97ee2641..065098007f 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -1455,6 +1455,12 @@
 #                    above node specified by @drive. If this option is not 
given,
 #                    a node name is autogenerated. (Since: 4.2)
 #
+# @x-max-workers: maximum of parallel requests for static data backup. This
+#                 doesn't influence copy-before-write operations. (Since: 4.3)
+#
+# @x-max-chunk: maximum chunk length for static data backup. This doesn't
+#               influence copy-before-write operations. (Since: 4.3)
+#
 # Note: @on-source-error and @on-target-error only affect background
 # I/O.  If an error occurs during a guest write request, the device's
 # rerror/werror actions will be used.
@@ -1469,7 +1475,8 @@
             '*on-source-error': 'BlockdevOnError',
             '*on-target-error': 'BlockdevOnError',
             '*auto-finalize': 'bool', '*auto-dismiss': 'bool',
-            '*filter-node-name': 'str' } }
+            '*filter-node-name': 'str', '*x-max-workers': 'int',
+            '*x-max-chunk': 'int64' } }
 
 ##
 # @DriveBackup:
diff --git a/include/block/block_int.h b/include/block/block_int.h
index dd033d0b37..55772f1591 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -1193,6 +1193,11 @@ void mirror_start(const char *job_id, BlockDriverState 
*bs,
  * @sync_mode: What parts of the disk image should be copied to the 
destination.
  * @sync_bitmap: The dirty bitmap if sync_mode is 'bitmap' or 'incremental'
  * @bitmap_mode: The bitmap synchronization policy to use.
+ * @max_workers: The limit for parallel requests for main backup loop.
+ *               Must be >= 1.
+ * @max_chunk: The limit for one IO operation length in main backup loop.
+ *             Must be not less than job cluster size or zero. Zero means no
+ *             specific limit.
  * @on_source_error: The action to take upon error reading from the source.
  * @on_target_error: The action to take upon error writing to the target.
  * @creation_flags: Flags that control the behavior of the Job lifetime.
@@ -1211,6 +1216,8 @@ BlockJob *backup_job_create(const char *job_id, 
BlockDriverState *bs,
                             BitmapSyncMode bitmap_mode,
                             bool compress,
                             const char *filter_node_name,
+                            int max_workers,
+                            int64_t max_chunk,
                             BlockdevOnError on_source_error,
                             BlockdevOnError on_target_error,
                             int creation_flags,
diff --git a/block/backup.c b/block/backup.c
index acab0d08da..606e4d8725 100644
--- a/block/backup.c
+++ b/block/backup.c
@@ -44,11 +44,16 @@ typedef struct BackupBlockJob {
     BlockdevOnError on_source_error;
     BlockdevOnError on_target_error;
     uint64_t len;
-    uint64_t bytes_read;
     int64_t cluster_size;
+    int max_workers;
+    int64_t max_chunk;
 
     BlockCopyState *bcs;
     BdrvDirtyBitmap *bcs_bitmap;
+
+    BlockCopyCallState *bcs_call;
+    int ret;
+    bool error_is_read;
 } BackupBlockJob;
 
 static const BlockJobDriver backup_job_driver;
@@ -57,7 +62,6 @@ static void backup_progress_bytes_callback(int64_t bytes, 
void *opaque)
 {
     BackupBlockJob *s = opaque;
 
-    s->bytes_read += bytes;
     job_progress_update(&s->common.job, bytes);
 }
 
@@ -69,25 +73,6 @@ static void backup_progress_reset_callback(void *opaque)
     job_progress_set_remaining(&s->common.job, estimate);
 }
 
-static int coroutine_fn backup_do_cow(BackupBlockJob *job,
-                                      int64_t offset, uint64_t bytes,
-                                      bool *error_is_read)
-{
-    int ret = 0;
-    int64_t start, end; /* bytes */
-
-    start = QEMU_ALIGN_DOWN(offset, job->cluster_size);
-    end = QEMU_ALIGN_UP(bytes + offset, job->cluster_size);
-
-    trace_backup_do_cow_enter(job, start, offset, bytes);
-
-    ret = block_copy(job->bcs, start, end - start, error_is_read);
-
-    trace_backup_do_cow_return(job, offset, bytes, ret);
-
-    return ret;
-}
-
 static void backup_cleanup_sync_bitmap(BackupBlockJob *job, int ret)
 {
     BdrvDirtyBitmap *bm;
@@ -166,54 +151,58 @@ static BlockErrorAction 
backup_error_action(BackupBlockJob *job,
     }
 }
 
-static bool coroutine_fn yield_and_check(BackupBlockJob *job)
+static void coroutine_fn backup_block_copy_callback(int ret, bool 
error_is_read,
+                                                    void *opaque)
 {
-    uint64_t delay_ns;
-
-    if (job_is_cancelled(&job->common.job)) {
-        return true;
-    }
-
-    /*
-     * We need to yield even for delay_ns = 0 so that bdrv_drain_all() can
-     * return. Without a yield, the VM would not reboot.
-     */
-    delay_ns = block_job_ratelimit_get_delay(&job->common, job->bytes_read);
-    job->bytes_read = 0;
-    job_sleep_ns(&job->common.job, delay_ns);
-
-    if (job_is_cancelled(&job->common.job)) {
-        return true;
-    }
+    BackupBlockJob *s = opaque;
 
-    return false;
+    s->bcs_call = NULL;
+    s->ret = ret;
+    s->error_is_read = error_is_read;
+    job_enter(&s->common.job);
 }
 
 static int coroutine_fn backup_loop(BackupBlockJob *job)
 {
-    bool error_is_read;
-    int64_t offset;
-    BdrvDirtyBitmapIter *bdbi;
-    int ret = 0;
+    while (true) { /* retry loop */
+        assert(!job->bcs_call);
+        job->bcs_call = block_copy_async(job->bcs, 0,
+                                         QEMU_ALIGN_UP(job->len,
+                                                       job->cluster_size),
+                                         true, job->max_workers, 
job->max_chunk,
+                                         backup_block_copy_callback);
 
-    bdbi = bdrv_dirty_iter_new(job->bcs_bitmap);
-    while ((offset = bdrv_dirty_iter_next(bdbi)) != -1) {
-        do {
-            if (yield_and_check(job)) {
-                goto out;
+        while (job->bcs_call && !job->common.job.cancelled) {
+            /* wait and handle pauses */
+
+            job_pause_point(&job->common.job);
+
+            if (job->bcs_call && !job->common.job.cancelled) {
+                job_yield(&job->common.job);
             }
-            ret = backup_do_cow(job, offset, job->cluster_size, 
&error_is_read);
-            if (ret < 0 && backup_error_action(job, error_is_read, -ret) ==
-                           BLOCK_ERROR_ACTION_REPORT)
-            {
-                goto out;
+        }
+
+        if (!job->bcs_call && job->ret == 0) {
+            /* Success */
+            return 0;
+        }
+
+        if (job->common.job.cancelled) {
+            if (job->bcs_call) {
+                block_copy_cancel(job->bcs_call);
             }
-        } while (ret < 0);
+            return 0;
+        }
+
+        if (!job->bcs_call && job->ret < 0 &&
+            (backup_error_action(job, job->error_is_read, -job->ret) ==
+             BLOCK_ERROR_ACTION_REPORT))
+        {
+            return job->ret;
+        }
     }
 
- out:
-    bdrv_dirty_iter_free(bdbi);
-    return ret;
+    g_assert_not_reached();
 }
 
 static void backup_init_bcs_bitmap(BackupBlockJob *job)
@@ -253,9 +242,14 @@ static int coroutine_fn backup_run(Job *job, Error **errp)
         int64_t count;
 
         for (offset = 0; offset < s->len; ) {
-            if (yield_and_check(s)) {
-                ret = -ECANCELED;
-                goto out;
+            if (job_is_cancelled(job)) {
+                return -ECANCELED;
+            }
+
+            job_pause_point(job);
+
+            if (job_is_cancelled(job)) {
+                return -ECANCELED;
             }
 
             ret = block_copy_reset_unallocated(s->bcs, offset, &count);
@@ -288,6 +282,25 @@ static int coroutine_fn backup_run(Job *job, Error **errp)
     return ret;
 }
 
+static void coroutine_fn backup_pause(Job *job)
+{
+    BackupBlockJob *s = container_of(job, BackupBlockJob, common.job);
+
+    if (s->bcs_call) {
+        block_copy_cancel(s->bcs_call);
+    }
+}
+
+static void coroutine_fn backup_set_speed(BlockJob *job, int64_t speed)
+{
+    BackupBlockJob *s = container_of(job, BackupBlockJob, common);
+
+    if (s->bcs) {
+        /* In block_job_create we yet don't have bcs */
+        block_copy_set_speed(s->bcs, s->bcs_call, speed);
+    }
+}
+
 static const BlockJobDriver backup_job_driver = {
     .job_driver = {
         .instance_size          = sizeof(BackupBlockJob),
@@ -298,7 +311,9 @@ static const BlockJobDriver backup_job_driver = {
         .commit                 = backup_commit,
         .abort                  = backup_abort,
         .clean                  = backup_clean,
-    }
+        .pause                  = backup_pause,
+    },
+    .set_speed = backup_set_speed,
 };
 
 static int64_t backup_calculate_cluster_size(BlockDriverState *target,
@@ -343,6 +358,8 @@ BlockJob *backup_job_create(const char *job_id, 
BlockDriverState *bs,
                   BitmapSyncMode bitmap_mode,
                   bool compress,
                   const char *filter_node_name,
+                  int max_workers,
+                  int64_t max_chunk,
                   BlockdevOnError on_source_error,
                   BlockdevOnError on_target_error,
                   int creation_flags,
@@ -363,6 +380,16 @@ BlockJob *backup_job_create(const char *job_id, 
BlockDriverState *bs,
     assert(sync_mode != MIRROR_SYNC_MODE_INCREMENTAL);
     assert(sync_bitmap || sync_mode != MIRROR_SYNC_MODE_BITMAP);
 
+    if (max_workers < 1) {
+        error_setg(errp, "At least one worker needed");
+        return NULL;
+    }
+
+    if (max_chunk < 0) {
+        error_setg(errp, "max-chunk is negative");
+        return NULL;
+    }
+
     if (bs == target) {
         error_setg(errp, "Source and target cannot be the same");
         return NULL;
@@ -418,6 +445,11 @@ BlockJob *backup_job_create(const char *job_id, 
BlockDriverState *bs,
     if (cluster_size < 0) {
         goto error;
     }
+    if (max_chunk && max_chunk < cluster_size) {
+        error_setg(errp, "Required max-chunk (%" PRIi64") is less than backup "
+                   "cluster size (%" PRIi64 ")", max_chunk, cluster_size);
+        return NULL;
+    }
 
     /*
      * If source is in backing chain of target assume that target is going to 
be
@@ -461,9 +493,12 @@ BlockJob *backup_job_create(const char *job_id, 
BlockDriverState *bs,
     job->bcs_bitmap = block_copy_dirty_bitmap(bcs);
     job->cluster_size = cluster_size;
     job->len = len;
+    job->max_workers = max_workers;
+    job->max_chunk = max_chunk;
 
     block_copy_set_callbacks(bcs, backup_progress_bytes_callback,
                              backup_progress_reset_callback, job);
+    block_copy_set_speed(bcs, NULL, speed);
 
     /* Required permissions are already taken by backup-top target */
     block_job_add_bdrv(&job->common, "target", target, 0, BLK_PERM_ALL,
diff --git a/block/replication.c b/block/replication.c
index 99532ce521..6ca47b616f 100644
--- a/block/replication.c
+++ b/block/replication.c
@@ -544,6 +544,7 @@ static void replication_start(ReplicationState *rs, 
ReplicationMode mode,
         s->backup_job = backup_job_create(
                                 NULL, s->secondary_disk->bs, 
s->hidden_disk->bs,
                                 0, MIRROR_SYNC_MODE_NONE, NULL, 0, false, NULL,
+                                0, 0,
                                 BLOCKDEV_ON_ERROR_REPORT,
                                 BLOCKDEV_ON_ERROR_REPORT, JOB_INTERNAL,
                                 backup_job_completed, bs, NULL, &local_err);
diff --git a/blockdev.c b/blockdev.c
index 8e029e9c01..8b1dc6782d 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -3502,6 +3502,9 @@ static BlockJob *do_backup_common(BackupCommon *backup,
     if (!backup->has_compress) {
         backup->compress = false;
     }
+    if (!backup->has_x_max_workers) {
+        backup->x_max_workers = 64;
+    }
 
     ret = bdrv_try_set_aio_context(target_bs, aio_context, errp);
     if (ret < 0) {
@@ -3581,6 +3584,8 @@ static BlockJob *do_backup_common(BackupCommon *backup,
                             backup->sync, bmap, backup->bitmap_mode,
                             backup->compress,
                             backup->filter_node_name,
+                            backup->x_max_workers,
+                            backup->x_max_chunk,
                             backup->on_source_error,
                             backup->on_target_error,
                             job_flags, NULL, NULL, txn, errp);
diff --git a/tests/qemu-iotests/129 b/tests/qemu-iotests/129
index cd6b9e9ce7..4a4a3760de 100755
--- a/tests/qemu-iotests/129
+++ b/tests/qemu-iotests/129
@@ -66,7 +66,6 @@ class TestStopWithBlockJob(iotests.QMPTestCase):
         result = self.vm.qmp("stop")
         self.assert_qmp(result, 'return', {})
         result = self.vm.qmp("query-block-jobs")
-        self.assert_qmp(result, 'return[0]/busy', True)
         self.assert_qmp(result, 'return[0]/ready', False)
 
     def test_drive_mirror(self):
@@ -77,7 +76,7 @@ class TestStopWithBlockJob(iotests.QMPTestCase):
     def test_drive_backup(self):
         self.do_test_stop("drive-backup", device="drive0",
                           target=self.target_img,
-                          sync="full")
+                          sync="full", speed=1024)
 
     def test_block_commit(self):
         self.do_test_stop("block-commit", device="drive0")
diff --git a/tests/qemu-iotests/185 b/tests/qemu-iotests/185
index 454ff600cc..7cd79052d9 100755
--- a/tests/qemu-iotests/185
+++ b/tests/qemu-iotests/185
@@ -182,7 +182,8 @@ _send_qemu_cmd $h \
                       'target': '$TEST_IMG.copy',
                       'format': '$IMGFMT',
                       'sync': 'full',
-                      'speed': 65536 } }" \
+                      'speed': 65536,
+                      'x-max-chunk': 65536 } }" \
     "return"
 
 # If we don't sleep here 'quit' command races with disk I/O
diff --git a/tests/qemu-iotests/219 b/tests/qemu-iotests/219
index e0c51662c0..e77ccf1b1b 100755
--- a/tests/qemu-iotests/219
+++ b/tests/qemu-iotests/219
@@ -223,6 +223,7 @@ with iotests.FilePath('disk.img') as disk_path, \
                 'target': copy_path,
                 'sync': 'full',
                 'speed': 262144,
+                'x-max-chunk': 65536,
                 'auto-finalize': auto_finalize,
                 'auto-dismiss': auto_dismiss,
             })
diff --git a/tests/qemu-iotests/257 b/tests/qemu-iotests/257
index a9828251cf..e86bd3d639 100755
--- a/tests/qemu-iotests/257
+++ b/tests/qemu-iotests/257
@@ -191,6 +191,7 @@ def blockdev_backup(vm, device, target, sync, **kwargs):
                         target=target,
                         sync=sync,
                         filter_node_name='backup-top',
+                        x_max_workers=1,
                         **kwargs)
     return result
 
diff --git a/tests/qemu-iotests/257.out b/tests/qemu-iotests/257.out
index 64dd460055..6997b56567 100644
--- a/tests/qemu-iotests/257.out
+++ b/tests/qemu-iotests/257.out
@@ -30,7 +30,7 @@ write -P0x76 0x3ff0000 0x10000
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_0", "sync": "full", 
"target": "ref_target_0"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_0", "sync": "full", 
"target": "ref_target_0", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_0", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -78,7 +78,7 @@ expecting 6 dirty sectors; have 6. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_1", "sync": "full", 
"target": "ref_target_1"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_1", "sync": "full", 
"target": "ref_target_1", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_1", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -92,7 +92,7 @@ expecting 6 dirty sectors; have 6. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "never", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_1", "sync": "bitmap", "target": 
"backup_target_1"}}
+{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "never", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_1", "sync": "bitmap", "target": 
"backup_target_1", "x-max-workers": 1}}
 {"return": {}}
 
 --- Write #2 ---
@@ -205,7 +205,7 @@ expecting 15 dirty sectors; have 15. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_2", "sync": "full", 
"target": "ref_target_2"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_2", "sync": "full", 
"target": "ref_target_2", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_2", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -219,7 +219,7 @@ expecting 15 dirty sectors; have 15. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "never", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_2", "sync": "bitmap", "target": 
"backup_target_2"}}
+{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "never", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_2", "sync": "bitmap", "target": 
"backup_target_2", "x-max-workers": 1}}
 {"return": {}}
 {"execute": "job-finalize", "arguments": {"id": "backup_2"}}
 {"return": {}}
@@ -290,7 +290,7 @@ write -P0x76 0x3ff0000 0x10000
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_0", "sync": "full", 
"target": "ref_target_0"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_0", "sync": "full", 
"target": "ref_target_0", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_0", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -338,7 +338,7 @@ expecting 6 dirty sectors; have 6. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_1", "sync": "full", 
"target": "ref_target_1"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_1", "sync": "full", 
"target": "ref_target_1", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_1", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -354,7 +354,7 @@ expecting 6 dirty sectors; have 6. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "never", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_1", "sync": "bitmap", "target": 
"backup_target_1"}}
+{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "never", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_1", "sync": "bitmap", "target": 
"backup_target_1", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"action": "report", "device": "backup_1", "operation": "read"}, 
"event": "BLOCK_JOB_ERROR", "timestamp": {"microseconds": "USECS", "seconds": 
"SECS"}}
 {"data": {"device": "backup_1", "error": "Input/output error", "len": 393216, 
"offset": 65536, "speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", 
"timestamp": {"microseconds": "USECS", "seconds": "SECS"}}
@@ -416,7 +416,7 @@ expecting 14 dirty sectors; have 14. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_2", "sync": "full", 
"target": "ref_target_2"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_2", "sync": "full", 
"target": "ref_target_2", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_2", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -430,7 +430,7 @@ expecting 14 dirty sectors; have 14. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "never", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_2", "sync": "bitmap", "target": 
"backup_target_2"}}
+{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "never", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_2", "sync": "bitmap", "target": 
"backup_target_2", "x-max-workers": 1}}
 {"return": {}}
 {"execute": "job-finalize", "arguments": {"id": "backup_2"}}
 {"return": {}}
@@ -501,7 +501,7 @@ write -P0x76 0x3ff0000 0x10000
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_0", "sync": "full", 
"target": "ref_target_0"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_0", "sync": "full", 
"target": "ref_target_0", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_0", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -549,7 +549,7 @@ expecting 6 dirty sectors; have 6. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_1", "sync": "full", 
"target": "ref_target_1"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_1", "sync": "full", 
"target": "ref_target_1", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_1", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -563,7 +563,7 @@ expecting 6 dirty sectors; have 6. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "never", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_1", "sync": "bitmap", "target": 
"backup_target_1"}}
+{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "never", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_1", "sync": "bitmap", "target": 
"backup_target_1", "x-max-workers": 1}}
 {"return": {}}
 
 --- Write #2 ---
@@ -676,7 +676,7 @@ expecting 15 dirty sectors; have 15. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_2", "sync": "full", 
"target": "ref_target_2"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_2", "sync": "full", 
"target": "ref_target_2", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_2", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -690,7 +690,7 @@ expecting 15 dirty sectors; have 15. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "never", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_2", "sync": "bitmap", "target": 
"backup_target_2"}}
+{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "never", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_2", "sync": "bitmap", "target": 
"backup_target_2", "x-max-workers": 1}}
 {"return": {}}
 {"execute": "job-finalize", "arguments": {"id": "backup_2"}}
 {"return": {}}
@@ -761,7 +761,7 @@ write -P0x76 0x3ff0000 0x10000
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_0", "sync": "full", 
"target": "ref_target_0"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_0", "sync": "full", 
"target": "ref_target_0", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_0", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -809,7 +809,7 @@ expecting 6 dirty sectors; have 6. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_1", "sync": "full", 
"target": "ref_target_1"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_1", "sync": "full", 
"target": "ref_target_1", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_1", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -823,7 +823,7 @@ expecting 6 dirty sectors; have 6. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_1", "sync": "bitmap", "target": 
"backup_target_1"}}
+{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_1", "sync": "bitmap", "target": 
"backup_target_1", "x-max-workers": 1}}
 {"return": {}}
 
 --- Write #2 ---
@@ -936,7 +936,7 @@ expecting 15 dirty sectors; have 15. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_2", "sync": "full", 
"target": "ref_target_2"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_2", "sync": "full", 
"target": "ref_target_2", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_2", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -950,7 +950,7 @@ expecting 15 dirty sectors; have 15. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_2", "sync": "bitmap", "target": 
"backup_target_2"}}
+{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_2", "sync": "bitmap", "target": 
"backup_target_2", "x-max-workers": 1}}
 {"return": {}}
 {"execute": "job-finalize", "arguments": {"id": "backup_2"}}
 {"return": {}}
@@ -1021,7 +1021,7 @@ write -P0x76 0x3ff0000 0x10000
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_0", "sync": "full", 
"target": "ref_target_0"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_0", "sync": "full", 
"target": "ref_target_0", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_0", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -1069,7 +1069,7 @@ expecting 6 dirty sectors; have 6. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_1", "sync": "full", 
"target": "ref_target_1"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_1", "sync": "full", 
"target": "ref_target_1", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_1", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -1085,7 +1085,7 @@ expecting 6 dirty sectors; have 6. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_1", "sync": "bitmap", "target": 
"backup_target_1"}}
+{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_1", "sync": "bitmap", "target": 
"backup_target_1", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"action": "report", "device": "backup_1", "operation": "read"}, 
"event": "BLOCK_JOB_ERROR", "timestamp": {"microseconds": "USECS", "seconds": 
"SECS"}}
 {"data": {"device": "backup_1", "error": "Input/output error", "len": 393216, 
"offset": 65536, "speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", 
"timestamp": {"microseconds": "USECS", "seconds": "SECS"}}
@@ -1147,7 +1147,7 @@ expecting 14 dirty sectors; have 14. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_2", "sync": "full", 
"target": "ref_target_2"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_2", "sync": "full", 
"target": "ref_target_2", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_2", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -1161,7 +1161,7 @@ expecting 14 dirty sectors; have 14. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_2", "sync": "bitmap", "target": 
"backup_target_2"}}
+{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_2", "sync": "bitmap", "target": 
"backup_target_2", "x-max-workers": 1}}
 {"return": {}}
 {"execute": "job-finalize", "arguments": {"id": "backup_2"}}
 {"return": {}}
@@ -1232,7 +1232,7 @@ write -P0x76 0x3ff0000 0x10000
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_0", "sync": "full", 
"target": "ref_target_0"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_0", "sync": "full", 
"target": "ref_target_0", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_0", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -1280,7 +1280,7 @@ expecting 6 dirty sectors; have 6. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_1", "sync": "full", 
"target": "ref_target_1"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_1", "sync": "full", 
"target": "ref_target_1", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_1", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -1294,7 +1294,7 @@ expecting 6 dirty sectors; have 6. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_1", "sync": "bitmap", "target": 
"backup_target_1"}}
+{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_1", "sync": "bitmap", "target": 
"backup_target_1", "x-max-workers": 1}}
 {"return": {}}
 
 --- Write #2 ---
@@ -1407,7 +1407,7 @@ expecting 12 dirty sectors; have 12. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_2", "sync": "full", 
"target": "ref_target_2"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_2", "sync": "full", 
"target": "ref_target_2", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_2", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -1421,7 +1421,7 @@ expecting 12 dirty sectors; have 12. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_2", "sync": "bitmap", "target": 
"backup_target_2"}}
+{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_2", "sync": "bitmap", "target": 
"backup_target_2", "x-max-workers": 1}}
 {"return": {}}
 {"execute": "job-finalize", "arguments": {"id": "backup_2"}}
 {"return": {}}
@@ -1492,7 +1492,7 @@ write -P0x76 0x3ff0000 0x10000
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_0", "sync": "full", 
"target": "ref_target_0"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_0", "sync": "full", 
"target": "ref_target_0", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_0", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -1540,7 +1540,7 @@ expecting 6 dirty sectors; have 6. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_1", "sync": "full", 
"target": "ref_target_1"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_1", "sync": "full", 
"target": "ref_target_1", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_1", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -1554,7 +1554,7 @@ expecting 6 dirty sectors; have 6. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "always", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_1", "sync": "bitmap", "target": 
"backup_target_1"}}
+{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "always", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_1", "sync": "bitmap", "target": 
"backup_target_1", "x-max-workers": 1}}
 {"return": {}}
 
 --- Write #2 ---
@@ -1667,7 +1667,7 @@ expecting 12 dirty sectors; have 12. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_2", "sync": "full", 
"target": "ref_target_2"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_2", "sync": "full", 
"target": "ref_target_2", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_2", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -1681,7 +1681,7 @@ expecting 12 dirty sectors; have 12. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "always", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_2", "sync": "bitmap", "target": 
"backup_target_2"}}
+{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "always", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_2", "sync": "bitmap", "target": 
"backup_target_2", "x-max-workers": 1}}
 {"return": {}}
 {"execute": "job-finalize", "arguments": {"id": "backup_2"}}
 {"return": {}}
@@ -1752,7 +1752,7 @@ write -P0x76 0x3ff0000 0x10000
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_0", "sync": "full", 
"target": "ref_target_0"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_0", "sync": "full", 
"target": "ref_target_0", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_0", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -1800,7 +1800,7 @@ expecting 6 dirty sectors; have 6. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_1", "sync": "full", 
"target": "ref_target_1"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_1", "sync": "full", 
"target": "ref_target_1", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_1", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -1816,7 +1816,7 @@ expecting 6 dirty sectors; have 6. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "always", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_1", "sync": "bitmap", "target": 
"backup_target_1"}}
+{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "always", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_1", "sync": "bitmap", "target": 
"backup_target_1", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"action": "report", "device": "backup_1", "operation": "read"}, 
"event": "BLOCK_JOB_ERROR", "timestamp": {"microseconds": "USECS", "seconds": 
"SECS"}}
 {"data": {"device": "backup_1", "error": "Input/output error", "len": 393216, 
"offset": 65536, "speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", 
"timestamp": {"microseconds": "USECS", "seconds": "SECS"}}
@@ -1878,7 +1878,7 @@ expecting 13 dirty sectors; have 13. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_2", "sync": "full", 
"target": "ref_target_2"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_2", "sync": "full", 
"target": "ref_target_2", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_2", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -1892,7 +1892,7 @@ expecting 13 dirty sectors; have 13. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "always", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_2", "sync": "bitmap", "target": 
"backup_target_2"}}
+{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "always", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_2", "sync": "bitmap", "target": 
"backup_target_2", "x-max-workers": 1}}
 {"return": {}}
 {"execute": "job-finalize", "arguments": {"id": "backup_2"}}
 {"return": {}}
@@ -1963,7 +1963,7 @@ write -P0x76 0x3ff0000 0x10000
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_0", "sync": "full", 
"target": "ref_target_0"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_0", "sync": "full", 
"target": "ref_target_0", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_0", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -2011,7 +2011,7 @@ expecting 6 dirty sectors; have 6. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_1", "sync": "full", 
"target": "ref_target_1"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_1", "sync": "full", 
"target": "ref_target_1", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_1", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -2025,7 +2025,7 @@ expecting 6 dirty sectors; have 6. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "always", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_1", "sync": "bitmap", "target": 
"backup_target_1"}}
+{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "always", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_1", "sync": "bitmap", "target": 
"backup_target_1", "x-max-workers": 1}}
 {"return": {}}
 
 --- Write #2 ---
@@ -2138,7 +2138,7 @@ expecting 12 dirty sectors; have 12. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_2", "sync": "full", 
"target": "ref_target_2"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_2", "sync": "full", 
"target": "ref_target_2", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_2", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -2152,7 +2152,7 @@ expecting 12 dirty sectors; have 12. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "always", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_2", "sync": "bitmap", "target": 
"backup_target_2"}}
+{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "always", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_2", "sync": "bitmap", "target": 
"backup_target_2", "x-max-workers": 1}}
 {"return": {}}
 {"execute": "job-finalize", "arguments": {"id": "backup_2"}}
 {"return": {}}
@@ -2223,7 +2223,7 @@ write -P0x76 0x3ff0000 0x10000
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_0", "sync": "full", 
"target": "ref_target_0"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_0", "sync": "full", 
"target": "ref_target_0", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_0", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -2271,7 +2271,7 @@ expecting 6 dirty sectors; have 6. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_1", "sync": "full", 
"target": "ref_target_1"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_1", "sync": "full", 
"target": "ref_target_1", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_1", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -2285,7 +2285,7 @@ expecting 6 dirty sectors; have 6. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_1", "sync": "full", "target": 
"backup_target_1"}}
+{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_1", "sync": "full", "target": 
"backup_target_1", "x-max-workers": 1}}
 {"return": {}}
 
 --- Write #2 ---
@@ -2398,7 +2398,7 @@ expecting 15 dirty sectors; have 15. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_2", "sync": "full", 
"target": "ref_target_2"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_2", "sync": "full", 
"target": "ref_target_2", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_2", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -2412,7 +2412,7 @@ expecting 15 dirty sectors; have 15. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_2", "sync": "bitmap", "target": 
"backup_target_2"}}
+{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_2", "sync": "bitmap", "target": 
"backup_target_2", "x-max-workers": 1}}
 {"return": {}}
 {"execute": "job-finalize", "arguments": {"id": "backup_2"}}
 {"return": {}}
@@ -2483,7 +2483,7 @@ write -P0x76 0x3ff0000 0x10000
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_0", "sync": "full", 
"target": "ref_target_0"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_0", "sync": "full", 
"target": "ref_target_0", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_0", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -2531,7 +2531,7 @@ expecting 6 dirty sectors; have 6. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_1", "sync": "full", 
"target": "ref_target_1"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_1", "sync": "full", 
"target": "ref_target_1", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_1", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -2547,7 +2547,7 @@ expecting 6 dirty sectors; have 6. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_1", "sync": "full", "target": 
"backup_target_1"}}
+{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_1", "sync": "full", "target": 
"backup_target_1", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"action": "report", "device": "backup_1", "operation": "read"}, 
"event": "BLOCK_JOB_ERROR", "timestamp": {"microseconds": "USECS", "seconds": 
"SECS"}}
 {"data": {"device": "backup_1", "error": "Input/output error", "len": 
67108864, "offset": 983040, "speed": 0, "type": "backup"}, "event": 
"BLOCK_JOB_COMPLETED", "timestamp": {"microseconds": "USECS", "seconds": 
"SECS"}}
@@ -2609,7 +2609,7 @@ expecting 14 dirty sectors; have 14. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_2", "sync": "full", 
"target": "ref_target_2"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_2", "sync": "full", 
"target": "ref_target_2", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_2", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -2623,7 +2623,7 @@ expecting 14 dirty sectors; have 14. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_2", "sync": "bitmap", "target": 
"backup_target_2"}}
+{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_2", "sync": "bitmap", "target": 
"backup_target_2", "x-max-workers": 1}}
 {"return": {}}
 {"execute": "job-finalize", "arguments": {"id": "backup_2"}}
 {"return": {}}
@@ -2694,7 +2694,7 @@ write -P0x76 0x3ff0000 0x10000
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_0", "sync": "full", 
"target": "ref_target_0"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_0", "sync": "full", 
"target": "ref_target_0", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_0", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -2742,7 +2742,7 @@ expecting 6 dirty sectors; have 6. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_1", "sync": "full", 
"target": "ref_target_1"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_1", "sync": "full", 
"target": "ref_target_1", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_1", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -2756,7 +2756,7 @@ expecting 6 dirty sectors; have 6. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_1", "sync": "full", "target": 
"backup_target_1"}}
+{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_1", "sync": "full", "target": 
"backup_target_1", "x-max-workers": 1}}
 {"return": {}}
 
 --- Write #2 ---
@@ -2869,7 +2869,7 @@ expecting 12 dirty sectors; have 12. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_2", "sync": "full", 
"target": "ref_target_2"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_2", "sync": "full", 
"target": "ref_target_2", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_2", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -2883,7 +2883,7 @@ expecting 12 dirty sectors; have 12. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_2", "sync": "bitmap", "target": 
"backup_target_2"}}
+{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_2", "sync": "bitmap", "target": 
"backup_target_2", "x-max-workers": 1}}
 {"return": {}}
 {"execute": "job-finalize", "arguments": {"id": "backup_2"}}
 {"return": {}}
@@ -2954,7 +2954,7 @@ write -P0x76 0x3ff0000 0x10000
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_0", "sync": "full", 
"target": "ref_target_0"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_0", "sync": "full", 
"target": "ref_target_0", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_0", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -3002,7 +3002,7 @@ expecting 6 dirty sectors; have 6. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_1", "sync": "full", 
"target": "ref_target_1"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_1", "sync": "full", 
"target": "ref_target_1", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_1", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -3016,7 +3016,7 @@ expecting 6 dirty sectors; have 6. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "always", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_1", "sync": "full", "target": 
"backup_target_1"}}
+{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "always", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_1", "sync": "full", "target": 
"backup_target_1", "x-max-workers": 1}}
 {"return": {}}
 
 --- Write #2 ---
@@ -3129,7 +3129,7 @@ expecting 12 dirty sectors; have 12. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_2", "sync": "full", 
"target": "ref_target_2"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_2", "sync": "full", 
"target": "ref_target_2", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_2", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -3143,7 +3143,7 @@ expecting 12 dirty sectors; have 12. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "always", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_2", "sync": "bitmap", "target": 
"backup_target_2"}}
+{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "always", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_2", "sync": "bitmap", "target": 
"backup_target_2", "x-max-workers": 1}}
 {"return": {}}
 {"execute": "job-finalize", "arguments": {"id": "backup_2"}}
 {"return": {}}
@@ -3214,7 +3214,7 @@ write -P0x76 0x3ff0000 0x10000
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_0", "sync": "full", 
"target": "ref_target_0"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_0", "sync": "full", 
"target": "ref_target_0", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_0", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -3262,7 +3262,7 @@ expecting 6 dirty sectors; have 6. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_1", "sync": "full", 
"target": "ref_target_1"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_1", "sync": "full", 
"target": "ref_target_1", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_1", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -3278,7 +3278,7 @@ expecting 6 dirty sectors; have 6. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "always", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_1", "sync": "full", "target": 
"backup_target_1"}}
+{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "always", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_1", "sync": "full", "target": 
"backup_target_1", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"action": "report", "device": "backup_1", "operation": "read"}, 
"event": "BLOCK_JOB_ERROR", "timestamp": {"microseconds": "USECS", "seconds": 
"SECS"}}
 {"data": {"device": "backup_1", "error": "Input/output error", "len": 
67108864, "offset": 983040, "speed": 0, "type": "backup"}, "event": 
"BLOCK_JOB_COMPLETED", "timestamp": {"microseconds": "USECS", "seconds": 
"SECS"}}
@@ -3340,7 +3340,7 @@ expecting 1014 dirty sectors; have 1014. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_2", "sync": "full", 
"target": "ref_target_2"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_2", "sync": "full", 
"target": "ref_target_2", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_2", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -3354,7 +3354,7 @@ expecting 1014 dirty sectors; have 1014. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "always", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_2", "sync": "bitmap", "target": 
"backup_target_2"}}
+{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "always", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_2", "sync": "bitmap", "target": 
"backup_target_2", "x-max-workers": 1}}
 {"return": {}}
 {"execute": "job-finalize", "arguments": {"id": "backup_2"}}
 {"return": {}}
@@ -3425,7 +3425,7 @@ write -P0x76 0x3ff0000 0x10000
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_0", "sync": "full", 
"target": "ref_target_0"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_0", "sync": "full", 
"target": "ref_target_0", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_0", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -3473,7 +3473,7 @@ expecting 6 dirty sectors; have 6. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_1", "sync": "full", 
"target": "ref_target_1"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_1", "sync": "full", 
"target": "ref_target_1", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_1", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -3487,7 +3487,7 @@ expecting 6 dirty sectors; have 6. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "always", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_1", "sync": "full", "target": 
"backup_target_1"}}
+{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "always", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_1", "sync": "full", "target": 
"backup_target_1", "x-max-workers": 1}}
 {"return": {}}
 
 --- Write #2 ---
@@ -3600,7 +3600,7 @@ expecting 12 dirty sectors; have 12. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_2", "sync": "full", 
"target": "ref_target_2"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_2", "sync": "full", 
"target": "ref_target_2", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_2", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -3614,7 +3614,7 @@ expecting 12 dirty sectors; have 12. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "always", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_2", "sync": "bitmap", "target": 
"backup_target_2"}}
+{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "always", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_2", "sync": "bitmap", "target": 
"backup_target_2", "x-max-workers": 1}}
 {"return": {}}
 {"execute": "job-finalize", "arguments": {"id": "backup_2"}}
 {"return": {}}
@@ -3685,7 +3685,7 @@ write -P0x76 0x3ff0000 0x10000
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_0", "sync": "full", 
"target": "ref_target_0"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_0", "sync": "full", 
"target": "ref_target_0", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_0", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -3733,7 +3733,7 @@ expecting 6 dirty sectors; have 6. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_1", "sync": "full", 
"target": "ref_target_1"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_1", "sync": "full", 
"target": "ref_target_1", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_1", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -3747,7 +3747,7 @@ expecting 6 dirty sectors; have 6. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_1", "sync": "top", "target": "backup_target_1"}}
+{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_1", "sync": "top", "target": "backup_target_1", 
"x-max-workers": 1}}
 {"return": {}}
 
 --- Write #2 ---
@@ -3860,7 +3860,7 @@ expecting 15 dirty sectors; have 15. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_2", "sync": "full", 
"target": "ref_target_2"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_2", "sync": "full", 
"target": "ref_target_2", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_2", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -3874,7 +3874,7 @@ expecting 15 dirty sectors; have 15. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_2", "sync": "bitmap", "target": 
"backup_target_2"}}
+{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_2", "sync": "bitmap", "target": 
"backup_target_2", "x-max-workers": 1}}
 {"return": {}}
 {"execute": "job-finalize", "arguments": {"id": "backup_2"}}
 {"return": {}}
@@ -3945,7 +3945,7 @@ write -P0x76 0x3ff0000 0x10000
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_0", "sync": "full", 
"target": "ref_target_0"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_0", "sync": "full", 
"target": "ref_target_0", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_0", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -3993,7 +3993,7 @@ expecting 6 dirty sectors; have 6. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_1", "sync": "full", 
"target": "ref_target_1"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_1", "sync": "full", 
"target": "ref_target_1", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_1", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -4009,7 +4009,7 @@ expecting 6 dirty sectors; have 6. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_1", "sync": "top", "target": "backup_target_1"}}
+{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_1", "sync": "top", "target": "backup_target_1", 
"x-max-workers": 1}}
 {"return": {}}
 {"data": {"action": "report", "device": "backup_1", "operation": "read"}, 
"event": "BLOCK_JOB_ERROR", "timestamp": {"microseconds": "USECS", "seconds": 
"SECS"}}
 {"data": {"device": "backup_1", "error": "Input/output error", "len": 458752, 
"offset": 65536, "speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", 
"timestamp": {"microseconds": "USECS", "seconds": "SECS"}}
@@ -4071,7 +4071,7 @@ expecting 14 dirty sectors; have 14. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_2", "sync": "full", 
"target": "ref_target_2"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_2", "sync": "full", 
"target": "ref_target_2", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_2", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -4085,7 +4085,7 @@ expecting 14 dirty sectors; have 14. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_2", "sync": "bitmap", "target": 
"backup_target_2"}}
+{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_2", "sync": "bitmap", "target": 
"backup_target_2", "x-max-workers": 1}}
 {"return": {}}
 {"execute": "job-finalize", "arguments": {"id": "backup_2"}}
 {"return": {}}
@@ -4156,7 +4156,7 @@ write -P0x76 0x3ff0000 0x10000
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_0", "sync": "full", 
"target": "ref_target_0"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_0", "sync": "full", 
"target": "ref_target_0", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_0", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -4204,7 +4204,7 @@ expecting 6 dirty sectors; have 6. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_1", "sync": "full", 
"target": "ref_target_1"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_1", "sync": "full", 
"target": "ref_target_1", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_1", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -4218,7 +4218,7 @@ expecting 6 dirty sectors; have 6. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_1", "sync": "top", "target": "backup_target_1"}}
+{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_1", "sync": "top", "target": "backup_target_1", 
"x-max-workers": 1}}
 {"return": {}}
 
 --- Write #2 ---
@@ -4331,7 +4331,7 @@ expecting 12 dirty sectors; have 12. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_2", "sync": "full", 
"target": "ref_target_2"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_2", "sync": "full", 
"target": "ref_target_2", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_2", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -4345,7 +4345,7 @@ expecting 12 dirty sectors; have 12. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_2", "sync": "bitmap", "target": 
"backup_target_2"}}
+{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_2", "sync": "bitmap", "target": 
"backup_target_2", "x-max-workers": 1}}
 {"return": {}}
 {"execute": "job-finalize", "arguments": {"id": "backup_2"}}
 {"return": {}}
@@ -4416,7 +4416,7 @@ write -P0x76 0x3ff0000 0x10000
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_0", "sync": "full", 
"target": "ref_target_0"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_0", "sync": "full", 
"target": "ref_target_0", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_0", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -4464,7 +4464,7 @@ expecting 6 dirty sectors; have 6. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_1", "sync": "full", 
"target": "ref_target_1"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_1", "sync": "full", 
"target": "ref_target_1", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_1", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -4478,7 +4478,7 @@ expecting 6 dirty sectors; have 6. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "always", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_1", "sync": "top", "target": "backup_target_1"}}
+{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "always", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_1", "sync": "top", "target": "backup_target_1", 
"x-max-workers": 1}}
 {"return": {}}
 
 --- Write #2 ---
@@ -4591,7 +4591,7 @@ expecting 12 dirty sectors; have 12. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_2", "sync": "full", 
"target": "ref_target_2"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_2", "sync": "full", 
"target": "ref_target_2", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_2", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -4605,7 +4605,7 @@ expecting 12 dirty sectors; have 12. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "always", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_2", "sync": "bitmap", "target": 
"backup_target_2"}}
+{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "always", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_2", "sync": "bitmap", "target": 
"backup_target_2", "x-max-workers": 1}}
 {"return": {}}
 {"execute": "job-finalize", "arguments": {"id": "backup_2"}}
 {"return": {}}
@@ -4676,7 +4676,7 @@ write -P0x76 0x3ff0000 0x10000
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_0", "sync": "full", 
"target": "ref_target_0"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_0", "sync": "full", 
"target": "ref_target_0", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_0", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -4724,7 +4724,7 @@ expecting 6 dirty sectors; have 6. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_1", "sync": "full", 
"target": "ref_target_1"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_1", "sync": "full", 
"target": "ref_target_1", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_1", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -4740,7 +4740,7 @@ expecting 6 dirty sectors; have 6. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "always", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_1", "sync": "top", "target": "backup_target_1"}}
+{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "always", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_1", "sync": "top", "target": "backup_target_1", 
"x-max-workers": 1}}
 {"return": {}}
 {"data": {"action": "report", "device": "backup_1", "operation": "read"}, 
"event": "BLOCK_JOB_ERROR", "timestamp": {"microseconds": "USECS", "seconds": 
"SECS"}}
 {"data": {"device": "backup_1", "error": "Input/output error", "len": 458752, 
"offset": 65536, "speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", 
"timestamp": {"microseconds": "USECS", "seconds": "SECS"}}
@@ -4802,7 +4802,7 @@ expecting 14 dirty sectors; have 14. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_2", "sync": "full", 
"target": "ref_target_2"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_2", "sync": "full", 
"target": "ref_target_2", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_2", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -4816,7 +4816,7 @@ expecting 14 dirty sectors; have 14. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "always", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_2", "sync": "bitmap", "target": 
"backup_target_2"}}
+{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "always", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_2", "sync": "bitmap", "target": 
"backup_target_2", "x-max-workers": 1}}
 {"return": {}}
 {"execute": "job-finalize", "arguments": {"id": "backup_2"}}
 {"return": {}}
@@ -4887,7 +4887,7 @@ write -P0x76 0x3ff0000 0x10000
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_0", "sync": "full", 
"target": "ref_target_0"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_0", "sync": "full", 
"target": "ref_target_0", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_0", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -4935,7 +4935,7 @@ expecting 6 dirty sectors; have 6. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_1", "sync": "full", 
"target": "ref_target_1"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_1", "sync": "full", 
"target": "ref_target_1", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_1", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -4949,7 +4949,7 @@ expecting 6 dirty sectors; have 6. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "always", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_1", "sync": "top", "target": "backup_target_1"}}
+{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "always", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_1", "sync": "top", "target": "backup_target_1", 
"x-max-workers": 1}}
 {"return": {}}
 
 --- Write #2 ---
@@ -5062,7 +5062,7 @@ expecting 12 dirty sectors; have 12. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_2", "sync": "full", 
"target": "ref_target_2"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "ref_backup_2", "sync": "full", 
"target": "ref_target_2", "x-max-workers": 1}}
 {"return": {}}
 {"data": {"device": "ref_backup_2", "len": 67108864, "offset": 67108864, 
"speed": 0, "type": "backup"}, "event": "BLOCK_JOB_COMPLETED", "timestamp": 
{"microseconds": "USECS", "seconds": "SECS"}}
 
@@ -5076,7 +5076,7 @@ expecting 12 dirty sectors; have 12. OK!
 {"execute": "job-dismiss", "arguments": {"id": "bdc-fmt-job"}}
 {"return": {}}
 {}
-{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "always", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_2", "sync": "bitmap", "target": 
"backup_target_2"}}
+{"execute": "blockdev-backup", "arguments": {"auto-finalize": false, "bitmap": 
"bitmap0", "bitmap-mode": "always", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "backup_2", "sync": "bitmap", "target": 
"backup_target_2", "x-max-workers": 1}}
 {"return": {}}
 {"execute": "job-finalize", "arguments": {"id": "backup_2"}}
 {"return": {}}
@@ -5139,155 +5139,155 @@ qemu_img compare "TEST_DIR/PID-img" 
"TEST_DIR/PID-fbackup2" ==> Identical, OK!
 
 -- Sync mode incremental tests --
 
-{"execute": "blockdev-backup", "arguments": {"bitmap-mode": "on-success", 
"device": "drive0", "filter-node-name": "backup-top", "job-id": "api_job", 
"sync": "incremental", "target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap-mode": "on-success", 
"device": "drive0", "filter-node-name": "backup-top", "job-id": "api_job", 
"sync": "incremental", "target": "backup_target", "x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "must provide a valid bitmap name 
for 'incremental' sync mode"}}
 
-{"execute": "blockdev-backup", "arguments": {"bitmap-mode": "always", 
"device": "drive0", "filter-node-name": "backup-top", "job-id": "api_job", 
"sync": "incremental", "target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap-mode": "always", 
"device": "drive0", "filter-node-name": "backup-top", "job-id": "api_job", 
"sync": "incremental", "target": "backup_target", "x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "must provide a valid bitmap name 
for 'incremental' sync mode"}}
 
-{"execute": "blockdev-backup", "arguments": {"bitmap-mode": "never", "device": 
"drive0", "filter-node-name": "backup-top", "job-id": "api_job", "sync": 
"incremental", "target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap-mode": "never", "device": 
"drive0", "filter-node-name": "backup-top", "job-id": "api_job", "sync": 
"incremental", "target": "backup_target", "x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "must provide a valid bitmap name 
for 'incremental' sync mode"}}
 
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "api_job", "sync": "incremental", 
"target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "api_job", "sync": "incremental", 
"target": "backup_target", "x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "must provide a valid bitmap name 
for 'incremental' sync mode"}}
 
-{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap404", 
"bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "api_job", "sync": "incremental", "target": 
"backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap404", 
"bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "api_job", "sync": "incremental", "target": 
"backup_target", "x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "Bitmap 'bitmap404' could not be 
found"}}
 
-{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap404", 
"bitmap-mode": "always", "device": "drive0", "filter-node-name": "backup-top", 
"job-id": "api_job", "sync": "incremental", "target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap404", 
"bitmap-mode": "always", "device": "drive0", "filter-node-name": "backup-top", 
"job-id": "api_job", "sync": "incremental", "target": "backup_target", 
"x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "Bitmap sync mode must be 
'on-success' when using sync mode 'incremental'"}}
 
-{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap404", 
"bitmap-mode": "never", "device": "drive0", "filter-node-name": "backup-top", 
"job-id": "api_job", "sync": "incremental", "target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap404", 
"bitmap-mode": "never", "device": "drive0", "filter-node-name": "backup-top", 
"job-id": "api_job", "sync": "incremental", "target": "backup_target", 
"x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "Bitmap sync mode must be 
'on-success' when using sync mode 'incremental'"}}
 
-{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap404", "device": 
"drive0", "filter-node-name": "backup-top", "job-id": "api_job", "sync": 
"incremental", "target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap404", "device": 
"drive0", "filter-node-name": "backup-top", "job-id": "api_job", "sync": 
"incremental", "target": "backup_target", "x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "Bitmap 'bitmap404' could not be 
found"}}
 
-{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap0", 
"bitmap-mode": "always", "device": "drive0", "filter-node-name": "backup-top", 
"job-id": "api_job", "sync": "incremental", "target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap0", 
"bitmap-mode": "always", "device": "drive0", "filter-node-name": "backup-top", 
"job-id": "api_job", "sync": "incremental", "target": "backup_target", 
"x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "Bitmap sync mode must be 
'on-success' when using sync mode 'incremental'"}}
 
-{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap0", 
"bitmap-mode": "never", "device": "drive0", "filter-node-name": "backup-top", 
"job-id": "api_job", "sync": "incremental", "target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap0", 
"bitmap-mode": "never", "device": "drive0", "filter-node-name": "backup-top", 
"job-id": "api_job", "sync": "incremental", "target": "backup_target", 
"x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "Bitmap sync mode must be 
'on-success' when using sync mode 'incremental'"}}
 
 -- Sync mode bitmap tests --
 
-{"execute": "blockdev-backup", "arguments": {"bitmap-mode": "on-success", 
"device": "drive0", "filter-node-name": "backup-top", "job-id": "api_job", 
"sync": "bitmap", "target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap-mode": "on-success", 
"device": "drive0", "filter-node-name": "backup-top", "job-id": "api_job", 
"sync": "bitmap", "target": "backup_target", "x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "must provide a valid bitmap name 
for 'bitmap' sync mode"}}
 
-{"execute": "blockdev-backup", "arguments": {"bitmap-mode": "always", 
"device": "drive0", "filter-node-name": "backup-top", "job-id": "api_job", 
"sync": "bitmap", "target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap-mode": "always", 
"device": "drive0", "filter-node-name": "backup-top", "job-id": "api_job", 
"sync": "bitmap", "target": "backup_target", "x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "must provide a valid bitmap name 
for 'bitmap' sync mode"}}
 
-{"execute": "blockdev-backup", "arguments": {"bitmap-mode": "never", "device": 
"drive0", "filter-node-name": "backup-top", "job-id": "api_job", "sync": 
"bitmap", "target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap-mode": "never", "device": 
"drive0", "filter-node-name": "backup-top", "job-id": "api_job", "sync": 
"bitmap", "target": "backup_target", "x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "must provide a valid bitmap name 
for 'bitmap' sync mode"}}
 
-{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "api_job", "sync": "bitmap", 
"target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"device": "drive0", 
"filter-node-name": "backup-top", "job-id": "api_job", "sync": "bitmap", 
"target": "backup_target", "x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "must provide a valid bitmap name 
for 'bitmap' sync mode"}}
 
-{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap404", 
"bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "api_job", "sync": "bitmap", "target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap404", 
"bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "api_job", "sync": "bitmap", "target": "backup_target", 
"x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "Bitmap 'bitmap404' could not be 
found"}}
 
-{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap404", 
"bitmap-mode": "always", "device": "drive0", "filter-node-name": "backup-top", 
"job-id": "api_job", "sync": "bitmap", "target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap404", 
"bitmap-mode": "always", "device": "drive0", "filter-node-name": "backup-top", 
"job-id": "api_job", "sync": "bitmap", "target": "backup_target", 
"x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "Bitmap 'bitmap404' could not be 
found"}}
 
-{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap404", 
"bitmap-mode": "never", "device": "drive0", "filter-node-name": "backup-top", 
"job-id": "api_job", "sync": "bitmap", "target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap404", 
"bitmap-mode": "never", "device": "drive0", "filter-node-name": "backup-top", 
"job-id": "api_job", "sync": "bitmap", "target": "backup_target", 
"x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "Bitmap 'bitmap404' could not be 
found"}}
 
-{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap404", "device": 
"drive0", "filter-node-name": "backup-top", "job-id": "api_job", "sync": 
"bitmap", "target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap404", "device": 
"drive0", "filter-node-name": "backup-top", "job-id": "api_job", "sync": 
"bitmap", "target": "backup_target", "x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "Bitmap 'bitmap404' could not be 
found"}}
 
-{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap0", "device": 
"drive0", "filter-node-name": "backup-top", "job-id": "api_job", "sync": 
"bitmap", "target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap0", "device": 
"drive0", "filter-node-name": "backup-top", "job-id": "api_job", "sync": 
"bitmap", "target": "backup_target", "x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "Bitmap sync mode must be given 
when providing a bitmap"}}
 
 -- Sync mode full tests --
 
-{"execute": "blockdev-backup", "arguments": {"bitmap-mode": "on-success", 
"device": "drive0", "filter-node-name": "backup-top", "job-id": "api_job", 
"sync": "full", "target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap-mode": "on-success", 
"device": "drive0", "filter-node-name": "backup-top", "job-id": "api_job", 
"sync": "full", "target": "backup_target", "x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "Cannot specify bitmap sync mode 
without a bitmap"}}
 
-{"execute": "blockdev-backup", "arguments": {"bitmap-mode": "always", 
"device": "drive0", "filter-node-name": "backup-top", "job-id": "api_job", 
"sync": "full", "target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap-mode": "always", 
"device": "drive0", "filter-node-name": "backup-top", "job-id": "api_job", 
"sync": "full", "target": "backup_target", "x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "Cannot specify bitmap sync mode 
without a bitmap"}}
 
-{"execute": "blockdev-backup", "arguments": {"bitmap-mode": "never", "device": 
"drive0", "filter-node-name": "backup-top", "job-id": "api_job", "sync": 
"full", "target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap-mode": "never", "device": 
"drive0", "filter-node-name": "backup-top", "job-id": "api_job", "sync": 
"full", "target": "backup_target", "x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "Cannot specify bitmap sync mode 
without a bitmap"}}
 
-{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap404", 
"bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "api_job", "sync": "full", "target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap404", 
"bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "api_job", "sync": "full", "target": "backup_target", 
"x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "Bitmap 'bitmap404' could not be 
found"}}
 
-{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap404", 
"bitmap-mode": "always", "device": "drive0", "filter-node-name": "backup-top", 
"job-id": "api_job", "sync": "full", "target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap404", 
"bitmap-mode": "always", "device": "drive0", "filter-node-name": "backup-top", 
"job-id": "api_job", "sync": "full", "target": "backup_target", 
"x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "Bitmap 'bitmap404' could not be 
found"}}
 
-{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap404", 
"bitmap-mode": "never", "device": "drive0", "filter-node-name": "backup-top", 
"job-id": "api_job", "sync": "full", "target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap404", 
"bitmap-mode": "never", "device": "drive0", "filter-node-name": "backup-top", 
"job-id": "api_job", "sync": "full", "target": "backup_target", 
"x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "Bitmap 'bitmap404' could not be 
found"}}
 
-{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap404", "device": 
"drive0", "filter-node-name": "backup-top", "job-id": "api_job", "sync": 
"full", "target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap404", "device": 
"drive0", "filter-node-name": "backup-top", "job-id": "api_job", "sync": 
"full", "target": "backup_target", "x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "Bitmap 'bitmap404' could not be 
found"}}
 
-{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap0", 
"bitmap-mode": "never", "device": "drive0", "filter-node-name": "backup-top", 
"job-id": "api_job", "sync": "full", "target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap0", 
"bitmap-mode": "never", "device": "drive0", "filter-node-name": "backup-top", 
"job-id": "api_job", "sync": "full", "target": "backup_target", 
"x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "Bitmap sync mode 'never' has no 
meaningful effect when combined with sync mode 'full'"}}
 
-{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap0", "device": 
"drive0", "filter-node-name": "backup-top", "job-id": "api_job", "sync": 
"full", "target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap0", "device": 
"drive0", "filter-node-name": "backup-top", "job-id": "api_job", "sync": 
"full", "target": "backup_target", "x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "Bitmap sync mode must be given 
when providing a bitmap"}}
 
 -- Sync mode top tests --
 
-{"execute": "blockdev-backup", "arguments": {"bitmap-mode": "on-success", 
"device": "drive0", "filter-node-name": "backup-top", "job-id": "api_job", 
"sync": "top", "target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap-mode": "on-success", 
"device": "drive0", "filter-node-name": "backup-top", "job-id": "api_job", 
"sync": "top", "target": "backup_target", "x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "Cannot specify bitmap sync mode 
without a bitmap"}}
 
-{"execute": "blockdev-backup", "arguments": {"bitmap-mode": "always", 
"device": "drive0", "filter-node-name": "backup-top", "job-id": "api_job", 
"sync": "top", "target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap-mode": "always", 
"device": "drive0", "filter-node-name": "backup-top", "job-id": "api_job", 
"sync": "top", "target": "backup_target", "x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "Cannot specify bitmap sync mode 
without a bitmap"}}
 
-{"execute": "blockdev-backup", "arguments": {"bitmap-mode": "never", "device": 
"drive0", "filter-node-name": "backup-top", "job-id": "api_job", "sync": "top", 
"target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap-mode": "never", "device": 
"drive0", "filter-node-name": "backup-top", "job-id": "api_job", "sync": "top", 
"target": "backup_target", "x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "Cannot specify bitmap sync mode 
without a bitmap"}}
 
-{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap404", 
"bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "api_job", "sync": "top", "target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap404", 
"bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "api_job", "sync": "top", "target": "backup_target", 
"x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "Bitmap 'bitmap404' could not be 
found"}}
 
-{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap404", 
"bitmap-mode": "always", "device": "drive0", "filter-node-name": "backup-top", 
"job-id": "api_job", "sync": "top", "target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap404", 
"bitmap-mode": "always", "device": "drive0", "filter-node-name": "backup-top", 
"job-id": "api_job", "sync": "top", "target": "backup_target", "x-max-workers": 
1}}
 {"error": {"class": "GenericError", "desc": "Bitmap 'bitmap404' could not be 
found"}}
 
-{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap404", 
"bitmap-mode": "never", "device": "drive0", "filter-node-name": "backup-top", 
"job-id": "api_job", "sync": "top", "target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap404", 
"bitmap-mode": "never", "device": "drive0", "filter-node-name": "backup-top", 
"job-id": "api_job", "sync": "top", "target": "backup_target", "x-max-workers": 
1}}
 {"error": {"class": "GenericError", "desc": "Bitmap 'bitmap404' could not be 
found"}}
 
-{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap404", "device": 
"drive0", "filter-node-name": "backup-top", "job-id": "api_job", "sync": "top", 
"target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap404", "device": 
"drive0", "filter-node-name": "backup-top", "job-id": "api_job", "sync": "top", 
"target": "backup_target", "x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "Bitmap 'bitmap404' could not be 
found"}}
 
-{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap0", 
"bitmap-mode": "never", "device": "drive0", "filter-node-name": "backup-top", 
"job-id": "api_job", "sync": "top", "target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap0", 
"bitmap-mode": "never", "device": "drive0", "filter-node-name": "backup-top", 
"job-id": "api_job", "sync": "top", "target": "backup_target", "x-max-workers": 
1}}
 {"error": {"class": "GenericError", "desc": "Bitmap sync mode 'never' has no 
meaningful effect when combined with sync mode 'top'"}}
 
-{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap0", "device": 
"drive0", "filter-node-name": "backup-top", "job-id": "api_job", "sync": "top", 
"target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap0", "device": 
"drive0", "filter-node-name": "backup-top", "job-id": "api_job", "sync": "top", 
"target": "backup_target", "x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "Bitmap sync mode must be given 
when providing a bitmap"}}
 
 -- Sync mode none tests --
 
-{"execute": "blockdev-backup", "arguments": {"bitmap-mode": "on-success", 
"device": "drive0", "filter-node-name": "backup-top", "job-id": "api_job", 
"sync": "none", "target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap-mode": "on-success", 
"device": "drive0", "filter-node-name": "backup-top", "job-id": "api_job", 
"sync": "none", "target": "backup_target", "x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "Cannot specify bitmap sync mode 
without a bitmap"}}
 
-{"execute": "blockdev-backup", "arguments": {"bitmap-mode": "always", 
"device": "drive0", "filter-node-name": "backup-top", "job-id": "api_job", 
"sync": "none", "target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap-mode": "always", 
"device": "drive0", "filter-node-name": "backup-top", "job-id": "api_job", 
"sync": "none", "target": "backup_target", "x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "Cannot specify bitmap sync mode 
without a bitmap"}}
 
-{"execute": "blockdev-backup", "arguments": {"bitmap-mode": "never", "device": 
"drive0", "filter-node-name": "backup-top", "job-id": "api_job", "sync": 
"none", "target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap-mode": "never", "device": 
"drive0", "filter-node-name": "backup-top", "job-id": "api_job", "sync": 
"none", "target": "backup_target", "x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "Cannot specify bitmap sync mode 
without a bitmap"}}
 
-{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap404", 
"bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "api_job", "sync": "none", "target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap404", 
"bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "api_job", "sync": "none", "target": "backup_target", 
"x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "Bitmap 'bitmap404' could not be 
found"}}
 
-{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap404", 
"bitmap-mode": "always", "device": "drive0", "filter-node-name": "backup-top", 
"job-id": "api_job", "sync": "none", "target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap404", 
"bitmap-mode": "always", "device": "drive0", "filter-node-name": "backup-top", 
"job-id": "api_job", "sync": "none", "target": "backup_target", 
"x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "Bitmap 'bitmap404' could not be 
found"}}
 
-{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap404", 
"bitmap-mode": "never", "device": "drive0", "filter-node-name": "backup-top", 
"job-id": "api_job", "sync": "none", "target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap404", 
"bitmap-mode": "never", "device": "drive0", "filter-node-name": "backup-top", 
"job-id": "api_job", "sync": "none", "target": "backup_target", 
"x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "Bitmap 'bitmap404' could not be 
found"}}
 
-{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap404", "device": 
"drive0", "filter-node-name": "backup-top", "job-id": "api_job", "sync": 
"none", "target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap404", "device": 
"drive0", "filter-node-name": "backup-top", "job-id": "api_job", "sync": 
"none", "target": "backup_target", "x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "Bitmap 'bitmap404' could not be 
found"}}
 
-{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap0", 
"bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "api_job", "sync": "none", "target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap0", 
"bitmap-mode": "on-success", "device": "drive0", "filter-node-name": 
"backup-top", "job-id": "api_job", "sync": "none", "target": "backup_target", 
"x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "sync mode 'none' does not produce 
meaningful bitmap outputs"}}
 
-{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap0", 
"bitmap-mode": "always", "device": "drive0", "filter-node-name": "backup-top", 
"job-id": "api_job", "sync": "none", "target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap0", 
"bitmap-mode": "always", "device": "drive0", "filter-node-name": "backup-top", 
"job-id": "api_job", "sync": "none", "target": "backup_target", 
"x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "sync mode 'none' does not produce 
meaningful bitmap outputs"}}
 
-{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap0", 
"bitmap-mode": "never", "device": "drive0", "filter-node-name": "backup-top", 
"job-id": "api_job", "sync": "none", "target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap0", 
"bitmap-mode": "never", "device": "drive0", "filter-node-name": "backup-top", 
"job-id": "api_job", "sync": "none", "target": "backup_target", 
"x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "sync mode 'none' does not produce 
meaningful bitmap outputs"}}
 
-{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap0", "device": 
"drive0", "filter-node-name": "backup-top", "job-id": "api_job", "sync": 
"none", "target": "backup_target"}}
+{"execute": "blockdev-backup", "arguments": {"bitmap": "bitmap0", "device": 
"drive0", "filter-node-name": "backup-top", "job-id": "api_job", "sync": 
"none", "target": "backup_target", "x-max-workers": 1}}
 {"error": {"class": "GenericError", "desc": "Bitmap sync mode must be given 
when providing a bitmap"}}
 
-- 
2.21.0