Bypass extent check for DEDB data sets

Enabling this option could result in improved performance in certain cases when multiple applications are simultaneously reading or writing the same disk extent ranges. For example, users of HSSP or HSRE with a Metro Mirror configuration may benefit from use of this feature.

Normally, when your program writes on DASD, the DASD subsystem performs extent serialization that prevents all access to that extent from all other programs on all systems while data is being written. This prevents other programs from reading and writing within the extent and from seeing certain types of incomplete updates. But if bypass extent check is enabled by coding BYPASS_EXTENT=Y in the FASTPATH section of the DFSDFxxx member, the DASD subsystem will serialize access to only one track at a time, not to the whole extent.

If you know that other programs might be reading the data set at the same time that your program is writing and that a mechanism exists within those other programs to avoid or tolerate potentially incomplete updates within an extent, then enabling bypass extent check could provide improved performance due to normal extent serialization being suspended.

This enhancement is delivered with APAR PH26898 (PTF UI91084).

Restrictions

When enabled, bypass extent check will function for all supported DEDB CI sizes except for:

• 3380 DASD: 12 KB, 16 KB, 24 KB, and 28 KB
• 3390 DASD: 20 KB and 28 KB

Coexistence considerations

It is acceptable to have bypass extent check enabled on one IMS and disabled on the other(s), but the bypass extent check option will not become active until it is enabled on all systems.

Log records

The following log records have been changed by this enhancement in IMS 15:

• Checkpoint record 4080: CHKRBEC flag is added to the CHKRSTF byte to indicate bypass extent check mode.

Changes to installing and defining IMS

After applying PTF UI91084 for APAR PH26898, you can enable the bypass extent check option by specifying BYPASS_EXTENT=Y in the FASTPATH section in the DFSDFxxx member. (The default is BYPASS_EXTENT=N).

Changes to troubleshooting for IMS

For a list of the messages and codes that are new or changed for this enhancement, see the IMS messages and codes row in the table in Documentation changes.

For a complete list of all of the new, changed, and deleted messages, and abend codes in IMS 15.5, see Message and code changes in IMS 15.5.

Documentation changes

The following table lists the publications that contain new or changed topics for the bypass extent check enhancement for DEDB data sets. Publications that are not impacted by this enhancement are not included in the table.