The PRELOAD option

The loading of one area takes place asynchronously with the loading of any others. The loading of an area is (or can be) concurrent with an application program's accesses to that area.

If the CI requested by the application program has been loaded into the data space, it is retrieved from the data space. If the requested CI has not yet been loaded into the data space, it is obtained from DASD and UOW locking is used to maintain data integrity.

The preload process for SHARELVL(2|3) VSO areas is similar to that of SHARELVL(0|1). Multiple preloads can be run concurrently, and also concurrent with application processing. The locking, however, is different. SHARELVL(2|3) Areas that are loaded into coupling facility structures use CI locking instead of UOW locking. The load process into the coupling facility is done one CI at a time.

If a read error occurs during preloading, an error message flags the error, but the preload process continues. If a subsequent application program call accesses a CI that was not loaded into the data space due to a read error, the CI request goes out to DASD. If the read error occurs again, the application program receives an A0 status code, just as with non-VSO applications. If instead the access to DASD is successful this time, the CI is loaded into the data space.