Ploop snapshot cannot be deleted with "Single delta, nothing to merge" error message

Original Publishing Date:
2022-04-13

Symptoms

Ploop snapshot cannot be deleted with the following error message:

~# prlctl snapshot-delete 333 -i 57ff7d22-ddce-4fd6-a8db-976094de6e55
Delete the snapshot...
Failed to delete snapshot: Operation failed. Failed to delete snapshot: Single delta, nothing to merge [38]
Failed to delete snapshot {57ff7d22-ddce-4fd6-a8db-976094de6e55}

Cause

The snapshot deletion process has been interrupted and DiskDescriptor.xml was not modified to reflect the changes.

Resolution

Confirm that only one delta is used:

~# grep 333 /sys/block/ploop*/pdelta/*/image
/sys/block/ploop26196/pdelta/0/image:/vz/private/333/root.hdd/root.hds

Check that the snapshot file is not held by any process:

~# ll /vz/private/333/root.hdd/
total 32879644
drwx------  4 root root        4096 Feb 11  2013 cache-private
drwxr-xr-x  2 root root        4096 Feb 11  2013 cache-root
-rw-r--r--  1 root root        1134 Mar  1 20:00 DiskDescriptor.xml
-rw-------  1 root root           0 Feb 11  2013 DiskDescriptor.xml.lck
-rw-------  1 root root 21077426176 Mar  5 02:49 root.hds
-rw-------  1 root root 12487491584 Mar  4 19:21 root.hds.{230c98c4-bdbb-4c0d-ad29-5269cce7366b}
drwx------  2 root root        4096 Apr 29  2013 root.hds.mnt
drwxr-xr-x 21 root root        4096 Apr 29  2013 templates
~# lsof 2>/dev/null | grep 230c98c4-bdbb-4c0d-ad29-5269cce7366b
~#

(In the example above you can also notice that modification date for root.hds file is more recent then for root.hds.{230c98c4-bdbb-4c0d-ad29-5269cce7366b})

Stop the affected container.
Create a backup copy of the directory /vz/private/CTID/root.hdd.

Modify <Storage> and <Snapshots> sections in /vz/private/CTID/root.hdd/DiskDescriptor.xml to exclude the snapshot. <Storage> and <Snapshots> sections in DiskDescriptor.xml of the container withiout any snapshots looks this way:

 <StorageData>
    <Storage>
      <Start>0</Start>
      <End>20971520</End>
      <Blocksize>2048</Blocksize>
      <Image>
        <GUID>{5fbaabe3-6958-40ff-92a7-860e329aab41}</GUID>
        <Type>Compressed</Type>
        <File>root.hds</File>
      </Image>
    </Storage>
  </StorageData>
  <Snapshots>
    <TopGUID>{5fbaabe3-6958-40ff-92a7-860e329aab41}</TopGUID>
    <Shot>
      <GUID>{5fbaabe3-6958-40ff-92a7-860e329aab41}</GUID>
      <ParentGUID>{00000000-0000-0000-0000-000000000000}</ParentGUID>
    </Shot>
  </Snapshots>

Delete the /vz/private/CTID/Snapshots.xml file.
Start the container.
Delete the unnecessary snapshot image (/vz/private/333/root.hdd/root.hds.{230c98c4-bdbb-4c0d-ad29-5269cce7366b}).