From: Vladimir Sementsov-Ogievskiy <vsement...@virtuozzo.com>
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsement...@virtuozzo.com>
Reviewed-by: Max Reitz <mre...@redhat.com>
Message-id: 20190920142056.12778-5-vsement...@virtuozzo.com
Signed-off-by: Max Reitz <mre...@redhat.com>
---
block/backup.c | 15 ++++++++++++---
1 file changed, 12 insertions(+), 3 deletions(-)
diff --git a/block/backup.c b/block/backup.c
index 98d7f7a905..ae28849660 100644
--- a/block/backup.c
+++ b/block/backup.c
@@ -747,9 +747,18 @@ BlockJob *backup_job_create(const char *job_id,
BlockDriverState *bs,
job->bitmap_mode = bitmap_mode;
/*
- * Set write flags:
- * 1. Detect image-fleecing (and similar) schemes
- * 2. Handle compression
+ * If source is in backing chain of target assume that target is going to
be
+ * used for "image fleecing", i.e. it should represent a kind of snapshot
of
+ * source at backup-start point in time. And target is going to be read by
+ * somebody (for example, used as NBD export) during backup job.
+ *
+ * In this case, we need to add BDRV_REQ_SERIALISING write flag to avoid
+ * intersection of backup writes and third party reads from target,
+ * otherwise reading from target we may occasionally read already updated
by
+ * guest data.
+ *
+ * For more information see commit f8d59dfb40bb and test
+ * tests/qemu-iotests/222
*/
job->write_flags =
(bdrv_chain_contains(target, bs) ? BDRV_REQ_SERIALISING : 0) |
--
2.21.0
