Share tape library resources

Because library devices can be shared among multiple systems and users, it is possible to run more jobs that use a tape library than there are resources in the library.

As users send commands to the tape library, requests to use a resource are sent to a tape resource manager. The request waits until a resource becomes available. When a resource is available, the job is assigned the resource to complete that step. How the system handles the requests depends on the Properties in iSeries™ Navigator that you specify for the tape library, or by using the Change Job Media Library Attributes (CHGJOBMLBA) command.

To specify the Properties for your tape library, do the following:

In iSeries Navigator expand My Connections > your iSeries server > Configuration and Service > Tape Devices > Hardware > Tape Libraries.
Right-click the library you want to work with and select Properties.
Select Options.
Specify the options you want:
- Tape resource selection priority
- Initial mount wait time
- End of volume mount wait time

Details: tape library properties

The order in which requests are given a resource is determined by the option you specify for Tape resource selection priority. The length of time a request will wait for a resource to become available is controlled by the length of time you specify for the request in Initial mount wait time and End of volume mount wait time. The time you specify in these properties can also be set in the tape library device description.

The character-based interface equivalent to the properties are the following parameters of the CHGJOBMLBA command:

Resource allocation priority (RSCALCPTY)
Initial mount wait time (INLMNTWAIT)
End of volume mount wait time (EOVMNTWAIT)

You can use these parameters as a default or for a particular job using the CHGJOBMLBA command or the Change Job Media Library Attributes (QTACJMA) API. These values are used when a request is first sent to the resource manager. After a request has been sent to the resource manager and is waiting to be processed, it can be monitored and changed using the Work with Media Library Resource Queue (WRKMLBRSCQ) command.

The system can automate how multiple library resources are shared. For example, assume ten save operations (SAVLIBs) are issued, with ten different cartridges, to a tape library device description (TAPMLB01). If TAPMLB01 has only four tape resources available to it, the first four requests are issued, one to each tape resource. The remaining six are placed on a queue and are issued as tape resources become available. The order in which they are placed on the queue is based on the RSCALCPTY parameter in the tape library device description. The resource manager eliminates any idle time between jobs due to early completion, from either changing data or job failure.

You can use Initial Mount Wait Time (INLMNTWAIT) value to alert you of a problem. In the previous example, the ten save operations are started at a given time and known, through estimates or benchmarking, to complete in six hours. Set the INLMNTWAIT time limit to six hours. If any of the save operations do not complete in six hours, an escape message is signaled to the jobs. A pager system can monitor for this message and page an operator to determine the necessary recovery actions.

Share tape library resources between systems

You can share tape library resources between more than one system. Tape resources can be set to ALLOCATED, UNPROTECTED, or DEALLOCATED. To better facilitate sharing between systems the UNPROTECTED status was added. When the tape resource is set to UNPROTECTED status, the tape resource is not assigned until it is needed. This prevents a tape resource from being assigned to a system that is not presently using it.

Share tape library resources between systems and on the same system

When sharing tape resources between systems and on the same system, each system has a resource manager with its queue controlled by the priority and time-out values. Between the systems, the tape resources are set to UNPROTECTED status. While sharing between systems does not have a priority concept, the UNPROTECTED status and the fact that tape resources are only assigned when they are in use allows the systems to effectively share the tape resources. The resource manager tries to get a resource by attempting to assign it. If another system has the resource, the job is placed back in the queue and waits. In a few seconds another assign is attempted. If the job now gets the resource, the tape operation continues.