Ubuntu 24.04 / kernel 6.8 — dm snapshot works but image capability bit (bit 2) not set in client binary

client
1

Environment:

  • UrBackup server: 2.5.31 (Infscape CE 1.14.0)

  • UrBackup client: 2.5.24

  • OS: Ubuntu 24.04.2 LTS, kernel 6.8.0

  • Snapshot method: Option 4 — device mapper snapshots

Issue:

After installing the Linux client and selecting option 4 (device mapper snapshots), the client reports capability_bits: 69632. Decoding this:

  • Bit 16 (dm snapshot support): :white_check_mark: set

  • Bit 2 (image backup capability): :cross_mark: not set

The server therefore never dispatches image backups to this client. File backups work correctly.

What we’ve confirmed:

  • dm_create_volume_snapshot succeeds — the snapshot device is created and mountable

  • The snapshot mechanism works; the client binary simply doesn’t advertise image capability to the server

  • dattobd is not available on kernel 6.8 (no Ubuntu 24.04 packages)

  • elastio-snap loads but has no UrBackup integration path

Secondary symptom: When snapshot attempts fail, the client leaves /.overlay_* files on the root filesystem. These are sparse files backed by the dm cow-storage device and can fill the root volume completely if left unchecked.

Question: Is there a known fix or workaround to force the image capability bit to be set for dm snapshots on kernel 6.8? Is this addressed in any client version after 2.5.24?

2

Does it set create_volume_snapshot in /usr/local/etc/urbackup/snapshot.cfg ?

3

Yes — create_volume_snapshot is set in snapshot.cfg:

create_volume_snapshot=/usr/local/share/urbackup/dm_create_snapshot
remove_volume_snapshot=/usr/local/share/urbackup/dm_remove_snapshot

The dm_create_snapshot script runs successfully and the snapshot device is created and mountable. The capability bit (bit 2) just isn’t being advertised to the server despite this.

4

Idk where you get this idea about this capability bit

5

Apologies — checking again now, capability_bits has changed from 69632 to 69644 since the server queued an image backup. Bit 2 is now set. We may have been looking at a stale value before the server had attempted to trigger an image backup. Is there a way to force an image backup now to test whether it proceeds?