Help

 

DFSSBIL0

DFSSBIL0 is the load module that contains the OSAM sequential buffering modules. OSAM sequential buffering can be a very fast way to read or update OSAM databases. Using OSAM sequential buffering is a tradeoff between CPU and elapsed time. For a small amount of CPU (and storage for buffers), the elapsed time for IMS jobs and utilities can be dramatically reduced.

DFSSBIL0 is also the name of the module within the load module that is invoked when OSAM sequential buffering is initialized. It stores the addresses of the other modules in this load module in an IMS control block (SCD), and loads the user exit DFSSBUX0 if it exists.

Hints

OSAM sequential buffering (OSB) can be very effective in reducing the elapsed time of IMS batch and BMP jobs, IMS utilities, and even long-running IMS transactions. When it is activated it checks, at a very low CPU cost, the access patterns to the OSAM databases. If it detects sequential accessing, it will start to asynchronously read ahead and store blocks in OSB buffer sets. Then, when the block is needed, it is moved from the OSB buffer to the OSAM buffer instead of waiting for an I/O.

It is worthwhile to activate OSB in a job and see if it is effective. By including a //DFSSTAT DD card in the JCL, IMS will print a number of statistics at the end of the job that indicate whether or not OSB was used productively.

Reference Sources

More information is available in the IMS DBSOURCE Library Data Set. See your IMS systems programmer.