snap-sync error: A device with UUID and subvolid was not found

[cmonty14]

Hi,
I have installed snap-sync on a SLES 12SP5.

However, there's an issue with running the first backup:

 # snap-sync --UUID b91e7771-06b9-49dc-a61c-8f2b4d729ac4 --subvolid 5

snap-sync version 0.6, Copyright (C) 2016-2019 Wes Barnett
snap-sync comes with ABSOLUTELY NO WARRANTY. This is free software, and you are welcome to redistribute it under certain conditions. See the license for more information.

Backup started: Starting backups to b91e7771-06b9-49dc-a61c-8f2b4d729ac4 subvolid=5...
==> ERROR: A device with UUID b91e7771-06b9-49dc-a61c-8f2b4d729ac4 and subvolid 5 was not found to be mounted, or it is not a BTRFS device.
Error: Check journal for more information.
Select a mounted BTRFS device on your local machine to backup to.
For more options, exit and run 'snap-sync -h'.
   1) /backup_rbd_lve_os (uuid=b91e7771-06b9-49dc-a61c-8f2b4d729ac4, subvolid=)
   0) Exit
Enter a number:

But I cannot identify an error with UUID.

 # blkid
/dev/sda1: UUID="75466633-932a-4005-852b-37d94a598654" TYPE="swap" PARTUUID="00025d86-01"
/dev/sda2: UUID="58b67250-be21-4c17-937c-b0f3603fcce3" UUID_SUB="eb6b7bfd-ce41-4b8d-abbe-e1dc67bbdbc4" TYPE="btrfs" PTTYPE="dos" PARTUUID="00025d86-02"
/dev/sdb: UUID="x7v7mP-Czpz-e7lh-PhvR-fJuJ-bnKP-NsWBsC" TYPE="LVM2_member"
/dev/rbd0: LABEL="Backup" UUID="de96cc20-e0d7-42c2-90cb-8bacdce5aa54" TYPE="xfs"
/dev/rbd1: LABEL="Backup-OS" UUID="b91e7771-06b9-49dc-a61c-8f2b4d729ac4" UUID_SUB="0ff3ca4c-885f-4ba9-9d4a-02671cea2f56" TYPE="btrfs"

mount | grep back
/dev/rbd0 on /backup_rbd_lve type xfs (rw,relatime,attr2,inode64,sunit=8192,swidth=8192,noquota)
/dev/rbd1 on /backup_rbd_lve_os type btrfs (rw,noatime,ssd,space_cache,subvolid=5,subvol=/)

For analysis I have attached output of bash -x snap-sync --UUID b91e7771-06b9-49dc-a61c-8f2b4d729ac4 --subvolid 5 to this ticket.
snap-sync-debug.txt

Please advise how to fix this error.

THX

[cmonty14]

Update:
I have identified the root cause.
When I mount the target device (formatted with BTRFS) it's using the toplevel subvolume (id=5).
/dev/rbd1 on /backup_rbd_lve_os type btrfs (rw,noatime,ssd,space_cache,subvolid=5,subvol=/)

But then the command btrfs subvolume show /backup_rbd_lve_os returns not expected result:

# btrfs subvolume show /backup_rbd_lve_os
/backup_rbd_lve_os is toplevel subvolume

As a workaround one could mount a subvolume with id=257 and execute command
snap-sync --UUID acaf6b0f-1dc2-4b6f-bae5-1b7d0e294cde --subvolid 257

Regards
Thomas