On 2017-12-17 08:52, Anand Jain wrote:
In two device configs of RAID1/RAID5 where one device can be missing
in the degraded mount, or in the configs such as four devices RAID6
where two devices can be missing, in these type of configs it can form
two separate set of devices where each of the set can be mounted without
the other set. And if user does that and writes the data, one set would
have to loose the data. As of now we just loose the data without the
user consent depending on the SB generation number which user won't have
any meaningful interpretations.
This patch introduces a new SB flag BTRFS_SUPER_FLAG_VOL_MOVED_ON which
will be set when RAID group is mounted with missing device, so when RAID
is mounted with no missing device and if all the devices contains this
flag then we know that each of these set are mounted without the other
set. In this scenario this patch will fail the mount so that user can
decide which set would they prefer to keep the data.
Now the procedure to assemble the disks would be to continue to mount
the good set first without the device set on which new data can be
ignored, and later run btrfs device scan to bring in the missing device
and complete the RAID group which then shall reset the flag
BTRFS_SUPER_FLAG_VOL_MOVED_ON.
Couple of thoughts on this:
1. This needs to go in _after_ the patches to allow the kernel to forget
devices. Otherwise, the only way to handle things is to physically
disconnect the device you don't want, mount the other one, and then
reconnect the disconnected device, because pretty much everything uses
udev, which by default scans for BTRFS on every device that gets
connected, and handling it like that is not an option for some people
(potentially for quite a few people).
2. How exactly is this supposed to work for people who don't have new
enough btrfs-progs to tell the kernel to forget the device? Ideally,
there should be some other way to tell the kernel which one to use
(perhaps a one-shot mount option?) without needing any userspace support
(and if that happens, then I retract my first comment). I'm not saying
that the currently proposed method is a bad solution, just that it's not
a complete solution from a usability perspective.
3. How does this get handled with volumes using the raid1 profile and
more than 2 devices? In that case, things get complicated fast.
Handling an array of N devices in raid1 mode where N-1 devices have this
flag set is in theory easy, except you have no verification that the N-1
devices were mounted separately or as a group (in the first case, the
user needs to pick one, in the second, it should automatically recover
since they all agree on the state of the filesystem).
4. Having some way to do this solely from btrfs-progs would be nice too
(so that you can just drop to a shell during boot, or drop to the
initramfs, and fix things without having to mess with device scanning),
but is secondary to the kernel-side IMHO.
Other than all of that, I do think this is a good idea. Being able to
more sanely inform the user about the situation and help them figure out
how to fix it correctly is something that's desperately needed here.
Signed-off-by: Anand Jain <anand.j...@oracle.com>
---
On top of kdave misc-next.
fs/btrfs/disk-io.c | 56 ++++++++++++++++++++++++++++++++++++++++-
fs/btrfs/volumes.c | 14 +++++++++--
include/uapi/linux/btrfs_tree.h | 1 +
3 files changed, 68 insertions(+), 3 deletions(-)
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index a3e9b74f6006..55bc6c846671 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -61,7 +61,8 @@
BTRFS_HEADER_FLAG_RELOC |\
BTRFS_SUPER_FLAG_ERROR |\
BTRFS_SUPER_FLAG_SEEDING |\
- BTRFS_SUPER_FLAG_METADUMP)
+ BTRFS_SUPER_FLAG_METADUMP|\
+ BTRFS_SUPER_FLAG_VOL_MOVED_ON)
static const struct extent_io_ops btree_extent_io_ops;
static void end_workqueue_fn(struct btrfs_work *work);
@@ -2383,6 +2384,43 @@ static int btrfs_read_roots(struct btrfs_fs_info
*fs_info)
return 0;
}
+bool volume_has_split_brain(struct btrfs_fs_info *fs_info)
+{
+ unsigned long devs_moved_on = 0;
+ struct btrfs_fs_devices *fs_devs = fs_info->fs_devices;
+ struct list_head *head = &fs_devs->devices;
+ struct btrfs_device *dev;
+
+again:
+ list_for_each_entry(dev, head, dev_list) {
+ struct buffer_head *bh;
+ struct btrfs_super_block *sb;
+
+ if (!dev->devid)
+ continue;
+
+ bh = btrfs_read_dev_super(dev->bdev);
+ if (IS_ERR(bh))
+ continue;
+
+ sb = (struct btrfs_super_block *)bh->b_data;
+ if (btrfs_super_flags(sb) & BTRFS_SUPER_FLAG_VOL_MOVED_ON)
+ devs_moved_on++;
+ brelse(bh);
+ }
+
+ fs_devs = fs_devs->seed;
+ if (fs_devs) {
+ head = &fs_devs->devices;
+ goto again;
+ }
+
+ if (devs_moved_on == fs_info->fs_devices->total_devices)
+ return true;
+ else
+ return false;
+}
+
int open_ctree(struct super_block *sb,
struct btrfs_fs_devices *fs_devices,
char *options)
@@ -2872,6 +2910,22 @@ int open_ctree(struct super_block *sb,
goto fail_sysfs;
}
+ if (fs_info->fs_devices->missing_devices) {
+ btrfs_set_super_flags(fs_info->super_copy,
+ fs_info->super_copy->flags |
+ BTRFS_SUPER_FLAG_VOL_MOVED_ON);
+ } else if (fs_info->super_copy->flags &
+ BTRFS_SUPER_FLAG_VOL_MOVED_ON) {
+ if (volume_has_split_brain(fs_info)) {
+ btrfs_err(fs_info,
+ "Detected 'moved_on' flag on all device");
+ goto fail_sysfs;
+ }
+ btrfs_set_super_flags(fs_info->super_copy,
+ fs_info->super_copy->flags &
+ ~BTRFS_SUPER_FLAG_VOL_MOVED_ON);
+ }
+
fs_info->cleaner_kthread = kthread_run(cleaner_kthread, tree_root,
"btrfs-cleaner");
if (IS_ERR(fs_info->cleaner_kthread))
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 33e814ef992f..c08b9b89e285 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -2057,8 +2057,13 @@ int btrfs_rm_device(struct btrfs_fs_info *fs_info, const
char *device_path,
device->fs_devices->num_devices--;
device->fs_devices->total_devices--;
- if (test_bit(BTRFS_DEV_STATE_MISSING, &device->dev_state))
+ if (test_bit(BTRFS_DEV_STATE_MISSING, &device->dev_state)) {
device->fs_devices->missing_devices--;
+ if (!device->fs_devices->missing_devices)
+ btrfs_set_super_flags(fs_info->super_copy,
+ fs_info->super_copy->flags &
+ ~BTRFS_SUPER_FLAG_VOL_MOVED_ON);
+ }
btrfs_assign_next_active_device(fs_info, device, NULL);
@@ -2132,8 +2137,13 @@ void btrfs_rm_dev_replace_remove_srcdev(struct btrfs_fs_info *fs_info,
list_del_rcu(&srcdev->dev_list);
list_del(&srcdev->dev_alloc_list);
fs_devices->num_devices--;
- if (test_bit(BTRFS_DEV_STATE_MISSING, &srcdev->dev_state))
+ if (test_bit(BTRFS_DEV_STATE_MISSING, &srcdev->dev_state)) {
fs_devices->missing_devices--;
+ if (!fs_devices->missing_devices)
+ btrfs_set_super_flags(fs_info->super_copy,
+ fs_info->super_copy->flags &
+ ~BTRFS_SUPER_FLAG_VOL_MOVED_ON);
+ }
if (test_bit(BTRFS_DEV_STATE_WRITEABLE, &srcdev->dev_state))
fs_devices->rw_devices--;
diff --git a/include/uapi/linux/btrfs_tree.h b/include/uapi/linux/btrfs_tree.h
index 6d6e5da51527..a9f625be0589 100644
--- a/include/uapi/linux/btrfs_tree.h
+++ b/include/uapi/linux/btrfs_tree.h
@@ -456,6 +456,7 @@ struct btrfs_free_space_header {
#define BTRFS_SUPER_FLAG_SEEDING (1ULL << 32)
#define BTRFS_SUPER_FLAG_METADUMP (1ULL << 33)
+#define BTRFS_SUPER_FLAG_VOL_MOVED_ON (1ULL << 36)
/*
--
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