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

1. 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
   

2. 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})

3. Stop the affected container.

4. Create a backup copy of the directory /vz/private/CTID/root.hdd.

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

    
       
         0
         20971520
         2048
         
           {5fbaabe3-6958-40ff-92a7-860e329aab41}
           Compressed
           root.hds
         
       
     
     
       {5fbaabe3-6958-40ff-92a7-860e329aab41}
       
         {5fbaabe3-6958-40ff-92a7-860e329aab41}
         {00000000-0000-0000-0000-000000000000}
       
     
   

6. Delete the /vz/private/CTID/Snapshots.xml file.

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