11.5 Specifying deferred write processing

Deferred write processing is a type of processing in which data is written to disk only when the number of updated pages reaches a specified value, instead of data being written each time a COMMIT statement is issued. The point when the number of updated pages reaches the specified value (as determined by HiRDB) is called the deferred write trigger. HiRDB determines the number of updated pages to be written to disk on the basis of the updated output page rate for deferred write trigger that is specified with the -w option of the pdbuffer operand in the system common definition. Deferred write processing cannot be performed for the following RDAREAs:

Organization of this section
(1) Effect of deferred write processing
(2) Specification
(3) Considerations
(4) Notes

(1) Effect of deferred write processing

Overloading caused by input/output processing can be reduced because data is not written to disk each time a COMMIT statement is issued.

(2) Specification

Either specify sync or nothing in the pd_dbsync_point operand. In addition, specify the updated output page rate for the deferred write trigger in the -w option of the pdbuffer operand.

(3) Considerations

  1. If a table or index in an RDAREA allocated to a global buffer is updated frequently, a low value should be set as the updated output page rate for deferred write trigger.
  2. If a global buffer is updated frequently but the same data is rarely updated, a high value should be set as the updated output page rate for deferred write trigger.
  3. After the HiRDB system has been started, the pdbufls command can be used to improve performance even further. In other words, each global buffer's update request hit rate, which is an edit item, should be checked and set as follows:
    • If the update request buffer hit rate is high, set the updated output page rate for deferred write trigger to a low value.
    • If the update request buffer hit rate is low, set the updated output page rate for deferred write trigger to a high value.

(4) Notes

If the updated output page rate for deferred write trigger is set to a higher value than is necessary, disk write operations occur more frequently during deferred write processing. This may cause a concurrently executing transaction to be placed in input/output wait status, with adverse effects on the response time.

On the other hand, if the updated output page rate for deferred write trigger is set too low, the number of pages to be written to the database increases, which may cause a concurrently executing transaction to be placed in input/output wait status, with adverse effects on the response time.