Mountpoint preemption in a library sharing environment

Answer

Mountpoint preemption occurs within a library sharing environment just as it does in a non-library sharing environment, the only difference being that the library manager is solely capable of preempting operations.

When a library client requests a tape mount it will inform the library manager of the mountpoint priority based on the operation that is being performed. For example, a database backup has a mountpoint priority of 1, whereas a client backup/archive operation has a mountpoint priority of 8. If the library manager receives a mount request from a library client and there are no available drives in the library, the library manager will then check the priority of the pending mount request to determine whether it is capable of preempting another operation. If the operation is capable of preempting another lower-priority operation, the library manager will then look at the priorities associated with all currently allocated mountpoints to determine whether any of these mountpoints are capable of being preempted. If one or more mountpoints are capable of being preempted, the library manager will then select the lowest priority operation to be preempted, even if the operation was initiated from a different library client. When preemption does occur, a message similar to the following will be seen in the activity log on the library client whose operation has been preempted:

ANR0492I All drives in use. Session 9999 for node BACKUPNODE (WinNT) being preempted by higher priority operation.

or

ANR1440I All drives in use. Process 99 being preempted by higher priority operation.

Mountpoint preemption can only occur within a single device class. That is, a higher-priority operation attempting to acquire a mountpoint in device class "A" would not be able to preempt a lower-priority operation that has already acquired a mountpoint in device class "B". Since preemption is initiated by the library manager, this means that the library clients would have to use a common device class on the library manager Server in order for preemption to occur.

Another point to consider is that not all operations are capable of preempting or being preempted. The AUDIT VOLUME process, for example, cannot be preempted by any other operations, nor is it capable of preempting any other operations. A detailed list of mountpoint priorities associated with each of the various operations can be found in the Tivoli Storage Manager Administrator's Guide.

Mountpoint preemption can be disabled by adding to the NOPREEMPT option to the dsmserv.opt file for the library manager Server. This option will prevent high-priority operations (eg. client restores) from preempting lower-priority backups, with the exception of database backups which will always preempt lower-priority operations.