COMPARE statement SNAPs

When a DL/I call does not produce the results you expect, you can use the COMPARE statement to compare the actual results of a call with the expected results. The normal output of this statement usually provides enough information to determine what is causing the problem.

When the output from a COMPARE statement does not provide enough information, you can use the SNAP option of the COMPARE statement to obtain additional diagnostic information. Specifically, the I/O buffer pool and the DL/I blocks are dumped. You can use the generated diagnostic output, in conjunction with IMS Version 15.4 Messages and Codes, Volume 3: IMSAbend Codes to determine the cause of the user abend you are diagnosing.

Attention: The COMPARE SNAP statement is a call to DL/I. Therefore, when a SNAP option is issued, some data in the captured area might be changed as a result. To prevent inadvertent change to data that is not involved in the problem, use a COMPARE SNAP statement only for the specific data that is involved in the problem.

Some control blocks are always dumped. Others are dumped only when you request them in the SNAP options.

The following control blocks are always dumped:

The SCD
The PST (save areas related to the current DL/I task are a part of the PST)
The Retrieve trace area

The following SNAP option requests dump the control blocks or buffers listed:

A request for the buffer pool dumps:
- OSAM buffer pool prefix and buffer pool, if present
- VSAM subpool prefix and buffer prefix and subpools
- Header for the DL/I, dispatcher, scheduler, and latch trace tables
- The DL/I trace table
- The dispatcher trace table
- The scheduler trace table
- The latch trace table
- Hierarchical direct (HD) trace table, if present
- Sequential buffering control blocks and buffer pools, if present
A request for the current DB PCB or all PSB-related control block dumps:
- Delete/replace work areas, when allocated
- ENQ/DEQ trace table, if present
- PSB and PSB work areas
- PCB information, including JCB, DSGs, and level table
- The block of SDBs, SDB expansion blocks, and generated SDBs
- DMB directories
- DMBs for the current PSB
- PNTs associated with partition DMBs
If you also requested buffers, a request for the current DB PCB or all PSB-related control block dumps:
- Any HISAM/QSAM buffers
- Any VSAM LRECs for each qualifying DSG
A request for the entire region, or subpools 0-127, dumps the entire region or the subpools.
A SNAP of the entire region or subpools is sent to a SNAP data set.

If the SNAP destination is the IMS log, the request is changed to a SNAP of all control blocks, regardless of other option specifications.

A region or subpool SNAP, when requested, appears before any additional SNAPs that were requested.

If the destination of the SNAP is the IMS log, you can select and format these records (type X'67FD') from the log by using the File Select and Formatting Print utility (DFSERA10) with formatting exit routine DFSERA30.