Re: incremental send/receive ERROR: cannot find parent subvolume

[Andrei Borzenkov <arvidjaar@gmail.com>]

27.02.2018 01:54, Emil.s пишет:
> Hello,
> 
> I'm trying to restore a subvolume from a backup, but I'm failing when
> I try to setup the replication chain again.
> 
> Previously I had disk A and B, where I was sending snapshots from A to
> B using "send -c /disk_a/1 /disk_a/2 | receive /disk_b" and so on.
> Now disk A failed, so I got a new disk: C.
> 
> First I sent the last backup from B to C by running "send disk_b/2 |
> receive /disk_c/".
> 
> Then my plan was to use disk B as the new main disk, so on disk B, I
> created a new snapshot by running "snapshot -r disk_b/2
> disk_b/my_volume".
> Now I want to send "disk_b/my_volume" to disk C, so that I can set
> disk_b/2 to RW and start using it again, but the last send fails with
> "ERROR: cannot find parent subvolume".
> 
> Disk B:
> root@tillberga: /backup #> btrfs subvol sh snapshots/user_data_2018-02-17/
> snapshots/user_data_2018-02-17
> Name: user_data_2018-02-17
> UUID: 93433261-d954-9f4b-8319-d450ae079a9a
> Parent UUID: 51180286-c202-c94c-b8f9-2ecc8d2b5b7c
> Received UUID: 014fc004-ae04-0148-9525-1bf556fd4d10
> Flags: readonly
> Snapshot(s):
> user_data_test
> 
> Disk C:
> root@hedemora: /btrfs #> btrfs subvol sh user_data_2018-02-17/
> user_data_2018-02-17
> Name: user_data_2018-02-17
> UUID: 35361429-a42c-594b-b390-51ffb9725324
> Parent UUID: -
> Received UUID: 93433261-d954-9f4b-8319-d450ae079a9a
> Flags: readonly
> Snapshot(s):
> 
> Disk B has UUID 93433261-d954-9f4b-8319-d450ae079a9a, and disk C has
> "Received UUID: 93433261-d954-9f4b-8319-d450ae079a9a". I think that's
> alright?
> 
> The new snapshot on disk B looks as following:
> root@tillberga: /backup #> btrfs subvol sh user_data_test
> user_data_test
> Name: user_data_test
> UUID: bf88000c-e3a6-434b-8f69-f5ce2174227e
> Parent UUID: 93433261-d954-9f4b-8319-d450ae079a9a
> Received UUID: 014fc004-ae04-0148-9525-1bf556fd4d10
> Flags: readonly
> Snapshot(s):
> 
> But when I'm trying to send it, I'm getting the following:
> root@tillberga: /backup #> btrfs send -v -c
> snapshots/user_data_2018-02-17 user_data_test | ssh user@hedemora
> "sudo btrfs receive -v /btrfs/"
> At subvol user_data_test
> BTRFS_IOC_SEND returned 0
> joining genl thread
> receiving snapshot user_data_test
> uuid=014fc004-ae04-0148-9525-1bf556fd4d10, ctransid=52373
> parent_uuid=014fc004-ae04-0148-9525-1bf556fd4d10,
> parent_ctransid=52373
> At snapshot user_data_test
> ERROR: cannot find parent subvolume
> 
> Note that the receiver says
> "parent_uuid=014fc004-ae04-0148-9525-1bf556fd4d10". Not really sure
> where that comes from, but disk B has the same, so maybe that's the
> UUID of the original snapshot on disk A?
> 


If "Received UUID" is set on a subvolume, "btrfs send" will forward it
and set on destination unchanged, instead of computing "correct"
Received UUID. This has been discussed several times.

> Is it possible to continue to send incremental snapshots between these
> two file systems, or must I do a full sync?

You would need to create writable clone on B first and start with this
clone; new writable clone won't have (misleading) Received UUID and you
start clean.