When setting a mount limit for a device class, you need to consider the number of storage devices connected to your system, whether you are using the simultaneous-write function, whether you are associating multiple device classes with a single library, and the number of processes that you want to run at the same time.
Do not specify a mount limit value that is greater than the number of associated available drives in your installation. If the server tries to mount as many volumes as specified by the mount limit and no drives are available for the required volume, an error occurs and client sessions may be terminated. (This does not apply when the DRIVES parameter is specified.)
Specify a mount limit value that provides a sufficient number of mount points to support writing data simultaneously to the primary storage pool and all associated copy storage pools and active-data pools.
A device class associated with a library can use any drive in the library that is compatible with the device class' device type. Because you can associate more than one device class with a library, a single drive in the library can be used by more than one device class. However, Tivoli® Storage Manager does not manage how a drive is shared among multiple device classes.
Tivoli Storage Manager automatically cancels some processes to run other, higher priority processes. If the server is using all available drives in a device class to complete higher priority processes, lower priority processes must wait until a drive becomes available. For example, Tivoli Storage Manager cancels the process for a client backing up directly to tape if the drive being used is needed for a server migration or tape reclamation process. Tivoli Storage Manager cancels a tape reclamation process if the drive being used is needed for a client restore operation. For additional information, see Preempting operations.
If processes are often canceled by other processes, consider whether you can make more drives available for Tivoli Storage Manager use. Otherwise, review your scheduling of operations to reduce the contention for drives.
This consideration also applies to the simultaneous-write function. You must have enough drives available to allow for a successful simultaneous-write operation.
To specify the maximum number of volumes that can be simultaneously mounted, use the MOUNTLIMIT parameter when you define the device class or update its definition.