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VARY Device Serialization

Whenever a tape device is shared among multiple systems, tape data corruption can occur if online processing for the device is occurring on one system while a job on another system is using the device. For example, Automatic Volume Recognition (AVR) during online processing may rewind a tape that is in use on another system. AVR is done as part of z/OS online processing. If a tape is found on a device during online processing, then AVR rewinds the tape and reads the label to find the VOLSER for the tape. If a VARY ONLINE is issued for a device on one system while the device is in use by a job on another system, then AVR during online processing may rewind the tape that is in use by the job on the other system, thus causing tape data corruption.

CA MIA VARY device serialization prevents the exposure outlined above. Device OFFLINE requests are managed by CA MIA as well to ensure the order of ONLINE and OFFLINE requests so that the resulting device states from such requests are set to the desired condition.

The CA MIA VARY device serialization provides granular device level serialization at online processing time. When CA MIA intercepts online processing locally for a device (VARY command, programmatic IEEVARYD call), it only allows online processing to occur if the device is not allocated or is not in the process of being allocated by a job or a SWAP on an external system. Conversely, while online processing is occurring locally for a device, CA MIA on external systems prevents jobs and SWAPs from allocating or going into the process of allocating the device. The device serialization is extended to be in effect throughout the entire online process. As mentioned, CA MIA controls VARY OFFLINE processing as well. This maintains the respective order of online and offline requests on the local system. Offline processing does not require the device level serialization, as does online processing; therefore, offline requests on the local system are not affected by any activity on an external system. However, an offline request might be forced to wait behind an outstanding local online request.

CA MIA cross-system VARY device serialization is performed through cross-system device group lock serialization. This aligns CA MIA VARY device serialization with tape allocation and SWAP serialization methods that also use cross-system device group lock serialization. By aligning device allocation, device swap, and device VARY to use the granular device level serialization technique, the performance and operational impacts of the VARY device serialization are minimized.