Input processing

When an application program issues a read request to a VSO area, IMS checks to see if the data is in the data space.

If the data is in the data space, it is copied from the data space into a common buffer and passed back to the application. If the data is not in the data space, IMS reads the CI from the area data set on DASD into a common buffer, copies the data into the data space, and passes the data back to the application.

For SHARELVL(2|3) VSO areas, Fast Path uses private buffer pools. Buffer lookaside is an option for these buffer pools. When a read request is issued against a SHARELVL(2|3) VSO area using a lookaside pool, a check is made to see if the requested data is in the pool. If the data is in the pool, a validity check to XES is made. If the data is valid, it is passed back to the application from the local buffer. If the data is not found in the local buffer pool or XES indicates that the data in the pool is not valid, the data is read from the coupling facility structure and passed to the application. When the buffer pool specifies the no-lookaside option, every request for data goes to the coupling facility.

For those areas that are defined as load-on-demand (using the VSO and NOPREL options), the first access to the CI is from DASD. The data is copied to the data space and then subsequent reads for this CI retrieve the data from the data space rather than from DASD. For those areas that are defined using the VSO and PRELOAD options, all access to CIs comes from the data space.

Whether the data comes from DASD or from the data space is transparent to the processing done by application programs.