The volsnap source errors are events that are listed in the Windows System event log. Such events usually contain relevant troubleshooting information as to why the shadow copy got dismounted and as a result causes the backups to fail.
You can refer to this article showcasing the error seen in Altaro.
Below you can find a couple of 'volsnap' events that we've encountered along with their solutions:
volsnap Event ID 25
The shadow copies of volume D: were deleted because the shadow copy storage could not grow in time. Consider reducing the IO load on the system or choose a shadow copy storage volume that is not being shadow copied.
This error is logged due to the source drive experiencing a high IO load and thereby it causes the shadow copy to dismount, as a result causing a backup failure. In this case you can choose to re-schedule the backup job when there is less IO on the source disk. In addition to that something that will help alleviate the IO on the source disk is placing the shadow copies onto another drive completely.
You’ll need to ensure that you have a disk with enough (10% of the original source) and it should also be a disk with on-per performance as the source.
You can run the following command on the host in order to place the shadow copy on another disk; drive letters need to be changed accordingly:
vssadmin add shadowstorage /For=D: /On=E: /MaxSize=UNBOUNDED vssadmin resize shadowstorage /For=D: /On=E: /MaxSize=UNBOUNDED
Adjusting the page file to 1.5 times the amount of RAM can also help the situation. Note that if you set the it to the maximum page file available, you will be required to restart the machine. Increases (not going up to the maximum) do not typically require a restart.
volsnap Event ID 16
The shadow copies of volume D: were aborted because volume D:, which contains shadow copy storage for this shadow copy, was force dismounted.
volsnap Event ID 14
The shadow copies of volume D: were aborted because of an IO failure on volume D:
These two events are usually coupled together. This usually points to a disk issue on the drive being referenced and there should be ‘Disk’ or ‘Ntfs’ events at the same time that give more information on the issue.
volsnap Event ID 24
There was insufficient disk space on volume D: to grow the shadow copy storage for shadow copies of D:. As a result of this failure all shadow copies of volume D: are at risk of being deleted.
volsnap Event ID 35
The shadow copies of volume D: were aborted because the shadow copy storage failed to grow.
These two events are usually coupled together. In this case it means that the shadow copy was dismounted due to insufficient disk space on the volume. Please ensure that you have at least 10% free space on the source drives and then run the backup again.
volsnap Event ID 36
The shadow copies of volume D: were aborted because the shadow copy storage could not grow due to a user imposed limit.
This particular volsnap error means that the current limit imposed is limiting the shadow copy from growing any larger and hence it’s causing the shadow copy to dismount and cause the backup to fail. To resolve this error you can run the following commands to expand the ShadowStorage; drive letters need to be changed accordingly:
vssadmin add shadowstorage /For=D: /On=D: /MaxSize=UNBOUNDED vssadmin resize shadowstorage /For=D: /On=D: /MaxSize=UNBOUNDED
Also note that in case you're using a CSV (Clustered Shared Volume), instead of a drive letter being listed in the event log, there will be an empty space.