Storage capacity for virtual tape libraries

Because virtual tape libraries (VTLs) do not have the physical limitations that real tape hardware does, their capacity for storage is more flexible.

The concept of storage capacity in a virtual tape library is different from capacity in physical tape hardware. In a physical tape library, each volume has a defined capacity, and the library's capacity is defined in terms of the total number of volumes in the library. The capacity of a VTL, alternatively, is defined in terms of total available disk space. You can increase or decrease the number and size of volumes on disk.

This variability affects what it means to run out of space in a VTL. For example, a volume in a VTL can run out of space before reaching its assigned capacity if the total underlying disk runs out of space. In this situation, the server can receive an end-of-volume message without any warning, resulting in backup failures.

When out-of-space errors and backup failures occur, disk space is usually still available in the VTL. It is hidden in volumes that are not in use. For example, volumes that are logically deleted or returned to scratch status in the IBM Storage Protect server are deleted only in the server database. The VTL is not notified, and the VTL maintains the full size of the volume as allocated in its capacity considerations.

To help prevent out-of-space errors, ensure that any SCSI library that you update to LIBTYPE=VTL is updated with the RELABELSCRATCH parameter set to YES. The RELABELSCRATCH option enables the server to overwrite the label for any volume that is deleted and to return the volume to scratch status in the library. The RELABELSCRATCH parameter defaults to YES for any library defined as a VTL.