[presztak]

Summary

Snapshotting a LINSTOR volume whose filesystem is mounted leaves the DRBD device permanently unmountable.
The kernel logs: drbd1001: Can't mount, blockdev is frozen.
The trigger is CreateVolumeSnapshot freezing the filesystem with fsfreeze and holding that freeze across the LINSTOR snapshot.

Reproducer

root@server11:~# incus launch images:debian/13 c1 --storage linstor --target server11
Launching c1
root@server11:~# incus storage volume create linstor d1
Storage volume d1 created
root@server11:~# incus storage volume attach linstor d1 c1 d1 /mnt/d1
root@server11:~# incus storage volume snapshot create linstor d1 snap0
root@server11:~# incus storage volume snapshot delete linstor d1 snap0
Storage volume snapshot snap0 deleted from d1
root@server11:~# incus stop c1
root@server11:~# incus start c1
Error: Failed to start device "d1": Failed mounting volume: Failed mounting custom storage volume "d1" on storage pool "linstor": Failed to mount "/dev/drbd1002" on "/var/lib/incus/storage-pools/linstor/custom/default_d1" using "ext4": device or resource busy
Try `incus info --show-log c1` for more info

Fix

Sync the filesystem rather than freezing it. This is a workaround. Bug also reported to LINSTOR: LINBIT/linstor-server#525