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Constraint Control Statements › Constraint Descriptions › DEFER: Deferring WRITEs

DEFER: Deferring WRITEs

Restriction: DEFER requires LSR to be effective. You can specify DEFER even if you do not specify LSR, and no error is generated. CA Hyper-Buf may convert strictly random access OPENs to use LSR buffers, and GVBDBFON cannot tell at initialization time how any particular cluster is accessed. If you specify DEFER when LSR is not in effect, normal VSAM buffer writes occur, and DEFER is ignored.

Default: No DEFER

Normally, VSAM writes buffered CIs to disk immediately when any data in the CI is altered. This is true for alterations caused by record insertions and record updates, including deletes.

Applications that insert or alter many logical records that reside in the same CI may avoid multiple WRITEs of the same CI by specifying DEFER.

DEFER causes VSAM to update the CI in the buffer and not write it to the disk until either the application forces a write (CLOSE, WRTBFR, and so forth) or until the buffer is required for a new CI to be read into storage.

This option can drastically reduce the number of I/O operations required to update a file randomly if the updates are clustered around specific keys in the file. This reduction in I/O can significantly improve the elapsed times for jobs that make multiple updates to the same CI.

Implications to Consider Before Using DEFER

Since it can be a long time (relative to CPU speeds) before the buffer is actually written to disk, it is possible that you will lose some data if the application abends or the processor dies (from a power failure, IPL, and so forth). In these situations, there may be data that has been updated in the buffer and not yet written to disk at the time of the failure. These updates may be lost.

If your processor never fails and your programs (including the operating system and its components) never abend, then you have no exposure. The less stable your system is, the greater your exposure.

For these reasons, you can exclude critical files (like journal and log files) from DEFER processing. If you do not request DEFER, normal write-as-you-go logic is used.