SBSNAP option

The SBSNAP option generates a SNAP of the relevant control blocks and areas involved in the calls of the OSAM buffer handler to the SB buffer handler.

Use the SBSNAP option when you receive a message saying that either Sequential Buffering:

  • Has been activated when you do not expect it to be
  • Has not been activated when you expect it to be activated

IMS monitors the physical I/O being done by individual applications and then uses SB I/O reference pattern-analysis algorithms to select the most efficient method of data access. When you suspect a problem with these algorithms, the SBSNAP option provides diagnostic output you can analyze. The information that is provided in the SNAPs provides an indication of why SB chose between issuing a random read of one single block and a sequential read of multiple consecutive blocks.

As a result of analyzing SBSNAP output, you might realize you need to reorganize the database, redesign the database, or set different thresholds for the SB definition. The SBSNAP option is also useful when you are tuning your usage of SB after you've installed IMS or migrated to a new version.

To activate the SBSNAP option, provide a SBSNAP control statement in the //DFSCTL file. (See IMS Version 15.5 System Definition for detailed information.)

SNAPs are written to the IMS log as type X'67EE' records. You can format and print these records by using the File Select and Formatting Print utility (DFSERA10) with exit routine DFSERA30.

The SBSNAP option often creates a very large amount of SNAP output. You might therefore decide to limit the SNAP to a specific short period of the application execution. To limit the SBSNAP option to one period of the application execution, use the START and STOP keywords on the SBSNAP control statement. The syntax for these keywords is: 
START=n STOP=
where n and m are the numbers of calls made to the SB buffer handler by the executing application.

To determine what values to use for n and m, look at the SPBSTCNB fields in the DL/I trace table and, if available, SNAP dumps (created by SBESNAP option). For each application, IMS maintains these call numbers in the SBPST, in the SBPSTCNB field. This field is periodically written to:

  • The X'6A' DL/I trace table entry
  • SNAPs that are created by the optional SBESNAP facility

Specifying START=n activates the SBSNAP option during the n the call to the SB buffer handler; specifying STOP=m deactivates the SBSNAP option during the mthe call to the SB buffer handler.