2 Cluster Shared Volumes filling up with shadow copies and I can't delete them.

Question

three node Server 2012 R2 cluster
2 CSVs
Veeam Backup software v7
We've been running out of space on our CSVs. I determined that the hidden System Volume Information directory is huge. It is huge because of large VSS files in the directory.
Path to CSVs
c:\ClusterStorage\Volume1
c:\ClusterStorage\Volume2
===========================================================================
List of VSS providers
vssadmin list providers
vssadmin 1.1 - Volume Shadow Copy Service administrative command-line tool
(C) Copyright 2001-2013 Microsoft Corp.
Provider name: 'SmVssProvider'
   Provider type: Hardware
   Provider Id: {24e4e8a3-69cb-4370-8b7b-c5276cd49765}
   Version: 11.10.0001
Provider name: 'Microsoft CSV Shadow Copy Helper Provider'
   Provider type: Software
   Provider Id: {26d02d81-6aac-4275-8504-b9c6edc5261d}
   Version: 1.0.0.1
Provider name: 'Microsoft CSV Shadow Copy Provider'
   Provider type: Software
   Provider Id: {400a2ff4-5eb1-44b0-8a05-1fcac0bcf9ff}
   Version: 1.0.0.1
Provider name: 'Microsoft File Share Shadow Copy provider'
   Provider type: Fileshare
   Provider Id: {89300202-3cec-4981-9171-19f59559e0f2}
   Version: 1.0.0.1
Provider name: 'Microsoft Software Shadow Copy provider 1.0'
   Provider type: System
   Provider Id: {b5946137-7b9f-4925-af80-51abd60b20d5}
   Version: 1.0.0.7
===========================================================================
Vssadmin list shadowstorage doesn't reference the location of my CSVs...
PS C:\Windows\system32> vssadmin list shadowstorage
vssadmin 1.1 - Volume Shadow Copy Service administrative command-line tool
(C) Copyright 2001-2013 Microsoft Corp.
Shadow Copy Storage association
   For volume: (C:)\\?\Volume{6bf32257-eccb-11e3-80b5-806e6f6e6963}\
   Shadow Copy Storage volume: (C:)\\?\Volume{6bf32257-eccb-11e3-80b5-806e6f6e6963}\
   Used Shadow Copy Storage space: 0 bytes (0%)
   Allocated Shadow Copy Storage space: 0 bytes (0%)
   Maximum Shadow Copy Storage space: 13.6 GB (10%)
============================================================================
PS C:\clusterstorage\volume1> mountvol
Creates, deletes, or lists a volume mount point.
MOUNTVOL [drive:]path VolumeName
MOUNTVOL [drive:]path /D
MOUNTVOL [drive:]path /L
MOUNTVOL [drive:]path /P
MOUNTVOL /R
MOUNTVOL /N
MOUNTVOL /E
    path        Specifies the existing NTFS directory where the mount
                point will reside.
    VolumeName  Specifies the volume name that is the target of the mount
                point.
    /D          Removes the volume mount point from the specified directory.
    /L          Lists the mounted volume name for the specified directory.
    /P          Removes the volume mount point from the specified directory,
                dismounts the volume, and makes the volume not mountable.
                You can make the volume mountable again by creating a volume
                mount point.
    /R          Removes volume mount point directories and registry settings
                for volumes that are no longer in the system.
    /N          Disables automatic mounting of new volumes.
    /E          Re-enables automatic mounting of new volumes.
Possible values for VolumeName along with current mount points are:
    \\?\Volume{6bf32256-eccb-11e3-80b5-806e6f6e6963}\
        *** NO MOUNT POINTS ***
    \\?\Volume{6bf32257-eccb-11e3-80b5-806e6f6e6963}\
        C:\
    \\?\Volume{bd46bae7-015c-4115-88ca-e6e9067fdea1}\
        C:\ClusterStorage\Volume2\
    \\?\Volume{caf6938a-5faa-4f3b-8e2b-e27d1498dedf}\
        C:\ClusterStorage\Volume1\
    \\?\Volume{6bf3225c-eccb-11e3-80b5-806e6f6e6963}\
        D:\
====================================================================

Answer 1

When veeam backup call VSS, it create cache files that at the end of backup should be purged. I suppose that in some case this process don't work properly and leave orphaned cache files.
it's possible that when you move CSV on another node these orphaned files will be purged.
from the disk management, right click on CSV, properties, can you see the shadow copies tab?