Re: send | receive: received snapshot is missing recent files

[A L <crimsoncottage@gmail.com>]

The problem can be that you have a Received UUID on the source volume. This breaks send-receive.

---- From: Dave <davestechshop@gmail.com> -- Sent: 2017-09-07 - 06:43 ----

> Here is more info and a possible (shocking) explanation. This
> aggregates my prior messages and it provides an almost complete set of
> steps to reproduce this problem.
> 
> Linux srv 4.9.41-1-lts #1 SMP Mon Aug 7 17:32:35 CEST 2017 x86_64 GNU/Linux
> btrfs-progs v4.12
> 
> My steps:
> 
> [root@srv]# sync
> [root@srv]# mkdir /home/.snapshots/test1
> [root@srv]# btrfs su sn -r /home/ /home/.snapshots/test1/
> Create a readonly snapshot of '/home/' in '/home/.snapshots/test1//home'
> [root@srv]# sync
> [root@srv]# mkdir /mnt/x5a/home/test1
> [root@srv]# btrfs send /home/.snapshots/test1/home/ | btrfs receive
> /mnt/x5a/home/test1/
> At subvol /home/.snapshots/test1/home/
> At subvol home
> [root@srv]# ls -la /mnt/x5a/home/test1/home/user1/
> NOTE: all recent files are present
> [root@srv]# ls -la /mnt/x5a/home/test1/home/user2/Documents/
> NOTE: all recent files are present
> [root@srv]# mkdir /home/.snapshots/test2
> [root@srv]# mkdir /mnt/x5a/home/test2
> [root@srv]# btrfs su sn -r /home/ /home/.snapshots/test2/
> Create a readonly snapshot of '/home/' in '/home/.snapshots/test2//home'
> [root@srv]# sync
> [root@srv]# btrfs send -p /home/.snapshots/test1/home/
> /home/.snapshots/test2/home/ | btrfs receive /mnt/x5a/home/test2/
> At subvol /home/.snapshots/test2/home/
> At snapshot home
> [root@srv]# ls -la /mnt/x5a/home/test2/home/user1/
> NOTE: all recent files are MISSING
> [root@srv]# ls -la /mnt/x5a/home/test2/home/user2/Documents/
> NOTE: all recent files are MISSING
> 
> Below I am including some rsync output to illustrate when a snapshot
> is missing files (or not):
> 
> [root@srv]# rsync -aniv /home/.snapshots/test1/home/
> /home/.snapshots/test2/home/
> sending incremental file list
> 
> sent 1,143,286 bytes  received 1,123 bytes  762,939.33 bytes/sec
> total size is 3,642,972,271  speedup is 3,183.28 (DRY RUN)
> 
> This indicates that these two subvolumes contain the same files, which
> they should because test2 is a snapshot of test1 without any changes
> to files, and it was not sent to another physical device.
> 
> The problem is when test2 is sent to another device as shown by the
> rsync results below.
> 
> [root@srv]# rsync -aniv /home/.snapshots/test2/home/ /mnt/x5a/home/test2/home/
> sending incremental file list
> .d..t...... ./
> .d..t...... user1/
>>f.st...... user1/.bash_history
>>f.st...... user1/.bashrc
>>f+++++++++ user1/test2017-09-06.txt
> ...
> and a long list of other missing files
> 
> The incrementally sent snapshot at /mnt/x5a/home/test2/home/ is
> missing all recent files (any files from the month of August or
> September), as my prior visual inspections had indicated. The same
> files are missing every time. There is no randomness to the missing
> data.
> 
> The problem does not happen for me if the receive command target is
> located on the same physical device as shown next. (However, I suspect
> there's more to it than that, as explained further below.)
> 
> [root@srv]# mkdir /home/.snapshots/test2rec
> [root@srv]# btrfs send -p /home/.snapshots/test1/home/
> /home/.snapshots/test2/home/ | btrfs receive
> /home/.snapshots/test2rec/
> At subvol /home/.snapshots/test2/home/
> 
> # rsync -aniv /home/.snapshots/test2/home/ /home/.snapshots/test2rec/home/
> sending incremental file list
> 
> sent 1,143,286 bytes  received 1,123 bytes  2,288,818.00 bytes/sec
> total size is 3,642,972,271  speedup is 3,183.28 (DRY RUN)
> 
> The above (as well as visual inspection of files) indicates that these
> two subvolumes contain the same files, which was not the case when the
> same command had a target located on another physical device. Of
> course, a snapshot which resides on the same physical device is not a
> very good backup. So I do need to send it to another device, but that
> results in missing files when the -p or -c options are used with btrfs
> send. (Non-incremental sending to another physical device does work.)
> 
> I can think of a couple possible explanations.
> 
> One is that there is a problem when using the -p or -c options with
> btrfs send when the target is another physical device. I suspect this
> is the actual explanation, however.
> 
> A second possibility is that the presence of prior existing snapshots
> at the target location (even if old and not referenced in any current
> btrfs command), can determine the outcome and final contents of an
> incremental send operation. I believe the info below suggests this to
> be the case.
> 
> [root@srv]# btrfs su show /home/.snapshots/test2/home/
> test2/home
>         Name:                   home
>         UUID:                   292e8bbf-a95f-2a4e-8280-129202d389dc
>         Parent UUID:            62418df6-a1f8-d74a-a152-11f519593053
>         Received UUID:          e00d5318-6efd-824e-ac91-f25efa5c2a74
>         Creation time:          2017-09-06 15:38:16 -0400
>         Subvolume ID:           2000
>         Generation:             5020
>         Gen at creation:        5020
>         Parent ID:              257
>         Top level ID:           257
>         Flags:                  readonly
>         Snapshot(s):
> 
> [root@srv]# btrfs su show /mnt/x5a/home/test1/home
> home/test1/home
>         Name:                   home
>         UUID:                   dc00b13d-f841-cf48-a169-aa61429a5679
>         Parent UUID:            -
>         Received UUID:          e00d5318-6efd-824e-ac91-f25efa5c2a74
>         Creation time:          2017-09-06 15:33:45 -0400
>         Subvolume ID:           656
>         Generation:             777
>         Gen at creation:        773
>         Parent ID:              257
>         Top level ID:           257
>         Flags:                  readonly
>         Snapshot(s):
> 
> [root@srv]# btrfs su show /mnt/x5a/home/test2/home/
> home/test2/home
>         Name:                   home
>         UUID:                   b01ab63f-17a1-f442-b9d4-ed12a0d057ea
>         Parent UUID:            8bf40f97-10e0-9f47-a281-1a0b21bbbad0
>         Received UUID:          e00d5318-6efd-824e-ac91-f25efa5c2a74
>         Creation time:          2017-09-06 15:39:51 -0400
>         Subvolume ID:           660
>         Generation:             779
>         Gen at creation:        779
>         Parent ID:              257
>         Top level ID:           257
>         Flags:                  readonly
>         Snapshot(s):
> 
> [root@srv]# btrfs su show /home/.snapshots/test2rec/home/
> test2rec/home
>         Name:                   home
>         UUID:                   bde1891d-1474-414f-b6ab-2a34c5af224e
>         Parent UUID:            62418df6-a1f8-d74a-a152-11f519593053
>         Received UUID:          e00d5318-6efd-824e-ac91-f25efa5c2a74
>         Creation time:          2017-09-06 17:36:19 -0400
>         Subvolume ID:           2003
>         Generation:             5027
>         Gen at creation:        5027
>         Parent ID:              257
>         Top level ID:           257
>         Flags:                  readonly
>         Snapshot(s):
> 
> Below, we have old almost forgotten snapshot (date 2017-07-21) on
> device /mnt/x5a/home with a Received UUID that matches the Received
> UUID of test snapshots that were newly created today. How? Why?
> 
> [root@thehulk home]# btrfs su show /mnt/x5a/home/107/snapshot
> home/107/snapshot
>         Name:                   snapshot
>         UUID:                   94d0bc47-dbf2-374e-b1c8-de06d729cde2
>         Parent UUID:            8bf40f97-10e0-9f47-a281-1a0b21bbbad0
>         Received UUID:          e00d5318-6efd-824e-ac91-f25efa5c2a74
>         Creation time:          2017-07-21 00:00:25 -0400
>         Subvolume ID:           433
>         Generation:             222
>         Gen at creation:        221
>         Parent ID:              257
>         Top level ID:           257
>         Flags:                  readonly
>         Snapshot(s):
> 
> If my guess is correct, btrfs has found this old snapshot and
> referenced it without me telling it to do so. The result is that the
> newly executed btrfs commands shown above have a totally unexpected
> result.
> 
> Today's new snapshot will not contain any files newer than 2017-07-21.
> Is this a known issue?
> 
> Refer back to the commands at the top of this message. I created a new
> snapshot and did a full (non-incremental) send to the target location
> (/mnt/x5a/home). Then I created a snapshot and did a send which only
> referenced the prior snapshot created today. Nowhere did I reference
> the ancient /mnt/x5a/home/107/snapshot. (Many prior snapshots exist at
> this backup location -- it was intended to hold a lot of them.) Yet,
> the very presence of /mnt/x5a/home/107/snapshot on the target device
> resulted in today's backup (and all recent backups) being worthless
> due to them missing all files since  2017-07-21.
> 
> These results are totally repeatable, given my set of existing
> backups. But it's bizarre to me. As I understand it, a staff person
> could transfer a btrfs snapshot to a target volume and it's mere
> presence there could make all subsequent backups (incremental sends)
> to that target volume invalid and useless. If that is true... wow.
> 
> Another interesting observation is that the device that contains the
> source snapshot, /home/.snapshots, also contains many, many prior
> snapshots, going back to when this system was first set up. Why do
> none of them cause a problem? Is it because I had never used
> /home/.snapshots as the target of a receive operation (until I did so
> today in testing the steps above)?
> 
> As far as repeating these steps, all this was totally repeatable for
> me as long as /mnt/x5a/home/107/snapshot existed on the target of the
> receive command (/mnt/x5a/home/). I do not know how to create such a
> "rogue" snapshot on purpose, but doing so may be key to reproducing my
> results.
> 
> Maybe somebody can explain to me what's really happening. How is it
> possible that an old snapshot created  2017-07-21 could have the same
> Received UUID as snapshots created today? And how could that fact lead
> to the result I'm seeing, which seems very serious. (Unexpected
> missing files from a backup which was completed without errors is
> pretty serious in my book.)
> 
> Most important question: how can we rely on automated incremental
> backups with btrfs send | receive given what I'm observing here
> (assuming my observations are roughly correct)?
> 
> Here's more info just to confirm that my results are not due to
> filesystem corruption.
> 
> running check on unmounted volume that contains /mnt/x5a/home/test2/home:
> [root@srv]# btrfs check -p /dev/mapper/x5a_luks
> Checking filesystem on /dev/mapper/x5a_luks
> UUID: 724f7cc1-41d8-456f-9fab-7ace457bd62a
> checking extents [o]
> checking free space cache [.]
> checking fs roots [o]
> checking csums
> checking root refs
> found 258178555904 bytes used, no error found
> total csum bytes: 250354776
> total tree bytes: 1752088576
> total fs tree bytes: 1308540928
> total extent tree bytes: 175161344
> btree space waste bytes: 215594634
> file data blocks allocated: 258634637312
>  referenced 292888985600
> 
> [root@srv]# btrfs fi show /mnt/x5a/
> Label: 'x5a_top'  uuid: 724f7cc1-41d8-456f-9fab-7ace457bd62a
>         Total devices 1 FS bytes used 240.45GiB
>         devid    1 size 4.55TiB used 244.07GiB path /dev/mapper/x5a_luks
> 
> [root@srv]# btrfs fi df /mnt/x5a/
> Data, single: total=239.01GiB, used=238.82GiB
> System, DUP: total=32.00MiB, used=48.00KiB
> Metadata, DUP: total=2.50GiB, used=1.63GiB
> GlobalReserve, single: total=422.73MiB, used=0.00B
> 
> # btrfs scrub status -d /mnt/x5a/
> scrub status for 724f7cc1-41d8-456f-9fab-7ace457bd62a
> scrub device /dev/mapper/x5a_luks (id 1) history
>         scrub started at Wed Sep  6 17:09:58 2017 and finished after 01:42:30
>         total bytes scrubbed: 242.08GiB with 0 errors
> --
> To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html