By default, the VSS Provider will attempt to convert standard volumes to master volumes on demand, automatically allocating and deallocating snap pools as needed. This mode of operation works best for turnkey configurations where the number of available snapshot licenses is less than the number of volumes and manual conversion of standard volumes to master volumes would be inconvenient. However, there are two potential drawbacks to the default behavior:
Insufficient free space on the vdisk
Creating snapshots on a standard volume may fail with the following error posted to the Applications event log:
calculateReserveSize(): The vdisk doesn't have sufficient space to create snap pool
This error occurs when the standard volume's vdisk does not contain enough free space to allocate a backing store which is at least 20% of the size of the volume, or 6GB, whichever is larger. To avoid this problem, find or create a snap pool on another vdisk with sufficient free space and convert the standard volume to a master volume using that snap pool instead. Placing the snap pool on a separate vdisk may also improve performance if the other vdisk is not subject to a heavy I/O load.
Snap pools are automatically deleted
In order to help manage limited disk space and snapshot licenses, the VSS Hardware Provider will, by default, automatically convert master volumes to standard volumes when the last snapshot for the volume is removed, and automatically delete snap pools when no snapshots refer to them.
However, this mode of operation may cause problems in environments where some manual management of snap pools is desired, or in SAN environments where multiple hosts are using VSS to create and delete snapshots concurrently on the same array.
To avoid these issues, the VSS Hardware Provider can be configured so that it does not attempt to automatically create or delete snap pools or convert standard volumes to master volumes and vice versa. Instead, the administrator must perform these steps manually.
Solution
To disable automatic conversion of standard volumes and automatic creation and deletion of snap pools, enter the following command at the Windows command prompt to change the provider's StandardVolumesAreSupported parameter value to 0:
reg add "HKLM\SYSTEM\CurrentControlSet\Services\HP StorageWorks VSS Hardware Provider\Parameters" /v StandardVolumesAreSupported /f /t REG_DWORD /d 0
To restore the default behavior, change the value of StandardVolumesAreSupported back to 2:
reg add "HKLM\SYSTEM\CurrentControlSet\Services\HP StorageWorks VSS Hardware Provider\Parameters" /v StandardVolumesAreSupported /f /t REG_DWORD /d 2
After changing the StandardVolumesAreSupported value, the VSS server must be stopped if it is currently running:
net stop VSS
net stop "HP StorageWorks VSS Hardware Provider"
Alternately, reboot the server to ensure that all the VSS components have been restarted.
As you stated that the Snap-pool can be created and the operation completes ok following that but then de-allocates the snap-pool, I believe that changing the VSS provider behavior and leaving the snap-pool should resolve the issue you are seeing
