The BS2000 function PAV is used to compensate for an idiosyncrasy of the /390 architecture, namely that IOs are serialized ahead of the device (which corresponds here to a logical volume). In other words, when a /390 server without PAV issues multiple IOs to the same volume, these must wait until each preceding IO has been concluded.
The consequence: increased software service times because of the wait time ahead of the device. Loads with the following are particularly prone to this:
high parallelism
and/or low cache hit rates.
In the case of the latter, cache hits which are actually quickly available can be significantly delayed by a preceding cache miss which has not yet been processed; the negative repercussions of the cache misses are thus multiplied.
PAV enables parallel disk access to be implemented on /390 servers. This permits the advantages of current channel and storage technologies to be utilized.
With PAV, multiple virtual devices (alias devices) can be assigned to one logical volume (base device). If a device is occupied, the IO can be started via one of the other virtual devices. The alias devices must be generated in BS2000; no intervention in the storage system is necessary.
PAV can be used as a static PAV, as a dynamic PAV (DPAV) or as FastDPAV.
Static PAV
In the case of static PAV, corresponding alias devices are generated and assigned in advance manually for heavily used devices. The decision regarding how many alias devices are assigned to which logical volumes is the responsibility of the system administrator.
Static PAV requires an analysis of the current usage of the volumes and foresighted planning with regard to the future device workload. In return, it enables targeted application for performance-critical productive data, as well as consistent performance which is tailored to requirements.
Dynamic PAV
In the case of dynamic PAV (DPAV), the IORM utility routine assigns the alias devices. The alias devices are assigned to the volumes with a heavy workload which profit most from this. The analysis and assignment take place in a 5-minute period here. DPAV enables the IO times to be enhanced for many loads with little overhead. However, because of its delayed mode of operation, DPAV is not suitable for dealing with short-term peak loads or for greatly changing loads, for example in the case of short-running batch jobs.
FastDPAV
Beginning with SU710, BS2000 supports an optimized DPAV, called FastDPAV: For a set of Volumes, connected via a set of paths with n channel paths per Volume, one FastDPAV base device is generated for each volume. Additionally a pool of FastDPAV alias devices without fixed mapping is generated for all volumes .
Handling an IO request, the BS2000 IO system (BS2IOS) uses first the base device and (if generated) static alias devices. If there are already active IOs for the base device and the static alias devices of this volume, BS2IOS will search for a suitable FastDPAV device and start the IO.
More in-depth information on PAV can be found in the “Introduction to System Administration” [10 (Related publications)].