Reports unsupported subvolume layout, even though the official script says it is fine

[jullanggit]

Issue

When i try to create a backup, i get the following error:
❯ sudo timeshift --check
Using system disk as snapshot device for creating snapshots in BTRFS mode
Mounted '/dev/dm-0 (nvme0n1p4)' at '/run/timeshift/2676/backup'
E: The system partition has an unsupported subvolume layout. Only ubuntu-type layouts with @ and @home subvolumes are currently supported.
Application will exit.

Full log

❯ sudo timeshift --check
Using system disk as snapshot device for creating snapshots in BTRFS mode
Mounted '/dev/dm-0 (nvme0n1p4)' at '/run/timeshift/2676/backup'
E: The system partition has an unsupported subvolume layout. Only ubuntu-type layouts with @ and @home subvolumes are currently supported.
Application will exit.

❯ grep -E '^[^#].+/\s+btrfs' /etc/fstab |
grep -oE 'subvol=[^,]+' |
cut -d= -f2 |
grep -qE '^/?@$' &&
echo 'OK' ||
echo 'Not OK'
OK

My fstab

LABEL=arch / btrfs rw,relatime,compress=zstd:3,ssd,space_cache=v2,subvol=/@,subvolid=256 0 0

LABEL=arch /.snapshots btrfs rw,relatime,compress=zstd:3,ssd,space_cache=v2,subvol=/@.snapshots,subvolid=260 0 0

LABEL=arch /home btrfs rw,relatime,compress=zstd:3,ssd,space_cache=v2,subvol=/@home,subvolid=257 0 0

LABEL=arch /var/cache btrfs rw,relatime,compress=zstd:3,ssd,space_cache=v2,subvol=/@cache,subvolid=258 0 0

LABEL=arch /var/log btrfs rw,relatime,compress=zstd:3,ssd,space_cache=v2,subvol=/@log,subvolid=259 0 0

LABEL=boot /boot ext4 rw,relatime 0 2

LABEL=efi /boot/efi vfat rw,relatime,fmask=0022,dmask=0022,codepage=437,iocharset=ascii,shortname=mixed,utf8,errors=remount-ro 0 2

/dev/mapper/swap none swap defaults 0 0

tmpfs /tmp tmpfs defaults,noatime,mode=1777 0 0

My system

• Arch Linux x86_64
• GNOME
• Timeshift 24.06.2

Conclusion

Any help regarding changes I'd have to make (for example to my fstab), as well as improving this issue (as i am inexperienced in creating them) are greatly appreciated.

[neitsab]

• the "check script" fails if fstab contains tabs instead of spaces in the wrong places
• the actual program fails for reasons I could not decipher:

% sudo timeshift --check --debug
D: Main()
D: 
D: Running Timeshift v24.06.6
D: 
D: Session log file: /var/log/timeshift/2025-01-22_17-04-36_backup.log
D: Distribution: arch
D: DIST_ID: arch
D: Main: check_dependencies()
D: Main: add_default_exclude_entries()
D: Main: add_default_exclude_entries(): exit
D: update_partitions()
D: df -T -B1
D: Device: get_disk_space_using_df(): 4
D: Device: get_mounted_filesystems_using_mtab(): 3
D: Device: get_filesystems(): 6
D: partition list updated
D: detect_system_devices()
D: /boot/efi is mapped to device: /dev/nvme0n1p1, UUID=1558-625E
D: /home is mapped to device: /dev/dm-0, UUID=a593c2fc-2cfe-4989-8cc4-1ebd7e8c3d84, subvol=@home
D: / is mapped to device: /dev/dm-0, UUID=a593c2fc-2cfe-4989-8cc4-1ebd7e8c3d84, subvol=@
D: Searching subvolume for system at path: /
D: Users: bastien root
D: Encrypted home users: 
D: Encrypted home dirs:

D: Encrypted private dirs:

D: Main: load_app_config()
D: Member not found in JSON object: snapshot_size
D: Member not found in JSON object: snapshot_count
App config loaded: /etc/timeshift/timeshift.json
D: IconManager: init()
D: found images directory: /usr/share/timeshift/images
D: Main(): ok
D: AppConsole: parse_arguments()
D: Main: initialize_repo()
D: backup_uuid=a593c2fc-2cfe-4989-8cc4-1ebd7e8c3d84
D: backup_parent_uuid=d6add504-4c62-4104-a961-97296db80045
Using system disk as snapshot device for creating snapshots in BTRFS mode
D: SnapshotRepo: from_device(): BTRFS
D: SnapshotRepo: init_from_device()
D: 
D: SnapshotRepo: unlock_and_mount_devices()
D: device=/dev/dm-0
D: SnapshotRepo: unlock_and_mount_device()
D: device=/dev/dm-0
D: Device: get_mounted_filesystems_using_mtab(): 3
D: ------------------
D: arg=a593c2fc-2cfe-4989-8cc4-1ebd7e8c3d84, device=/dev/dm-0
D: /home
D: /
D: ------------------
D: Device: get_mounted_filesystems_using_mtab(): 3
Mounted '/dev/dm-0 (nvme0n1p2)' at '/run/timeshift/1158359/backup'
D: Searching subvolume for system at path: /run/timeshift/1158359/backup/@
D: SnapshotRepo: load_snapshots()
D: SnapshotRepo: unlock_and_mount_device(): exit
D: Selected snapshot device: /dev/dm-0
D: Free space: 1.7 TB
D: SnapshotRepo: check_status()
D: SnapshotRepo: available()
D: SnapshotRepo: has_btrfs_system()
D: root_path=/run/timeshift/1158359/backup/@
D: btrfs_mode=true
D: is_available: ok
D: SnapshotRepo: has_snapshots()
D: SnapshotRepo: has_space() - 0 required (0 B)
D: df -T -B1 '/dev/dm-0'
D: Device: get_disk_space_using_df(): 1
D: no snapshots
D: SnapshotRepo: check_status(): exit
D: SnapshotRepo: init_from_device(): exit
D: Main: initialize_repo(): exit
D: AppConsole: start_application()
D: Main: create_snapshot()
D: check_btrfs_layout_system()
E: The system partition has an unsupported subvolume layout. Only ubuntu-type layouts with @ and @home subvolumes are currently supported.

[mskiptr]

I'm also seeing this error. I just installed Linux Mint using lmde-6-cinnamon-64bit.iso, but before rebooting I've adjusted /etc/fstab to use zstd:1 compression by default and defragged the filesystem to initially compress everything with zstd:15 (in the hope this increases read speeds for this HDD). While editing the fstab I cleaned it up a bit: got rid of the pointless /proc mount, replaced filesystem UUIDs with LABELs, removed 0 0 from btrfs lines, got rid of comments | empty lines and aligned the columns.

Sadly, replacing tabs with spaces (using expand /etc/fstab | sudo tee /etc/fstab) did not help in my case.

edit: Replacing all whitespace with just tabs or spaces (no alignment) does not work either. Nor does adding the 0 0s back in.

[mskiptr]

rel and most likely the cause: https://forum.manjaro.org/t/timeshift-is-reporting-an-error-saying-i-dont-have-an-ubuntu-subvolume-layout-but-my-layout-seems-fine/172176/6

I thought I had tried this already but perhaps not. So I'll see if switching back to UUIDs helps and report back in a few days most likely.

[mskiptr]

Alright, by staring at the code a bunch I've now found the offending line. The checks here, will consider anything mounted via a LABEL, a PARTLABEL or a PARTUUID to be a "system directory". And that's relevant when building up the sys_subvolumes hashmap, which in turn is what ultimately signals whether the @ and @home subvolumes are present. Otherwise the code path does seem to be mostly alright (besides a few strings which would be set to empty). The rest of the code however appears to assume it's working with either UUIDs or device paths and it also has some special /dev/mapper/ handling. I am not sure how well it would deal with both of those missing.

@teejee2008 Do you remember enough of the design to tell us if this naive fix is enough and won't just break everything?

[DJCrashdummy]

this is a duplicate of #142. but #142 contains just a few additional links.