Write operations

Write operations are usually performed concurrently with user requests.

Updated pages are queued by data set until they are written when one of the following events occurs:

  • A checkpoint is taken
  • The percentage of updated pages in a buffer pool for a single data set exceeds a preset limit called the vertical deferred write threshold (VDWQT)
  • The percentage of unavailable pages in a buffer pool exceeds a preset limit called the deferred write threshold (DWQT)

The following table lists how many pages DB2® can write in a single I/O operation.

Table 1. Number of pages that DB2 can write in a single I/O operation
Page size Number of pages
4 KB 32
8 KB 16
16 KB 8
32 KB 4

The following table lists how many pages DB2 can write in a single utility I/O operation. If the number of buffers is large enough, DB2 can write twice as many pages for each I/O operation for a utility write.

Table 2. The number of pages that DB2 can write for a single I/O operation for utility writes
Page Size Number of buffers Number of pages
4 KB BP ≥ 80,000 128
BP < 80,000 64
8 KB BP ≥ 40,000 64
BP < 40,000 32
16 KB BP ≥ 20,000 32
BP < 20,000 16
32 KB BP ≥ 10,000 16
BP < 10,000 8

As with utility write operations, DB2 can write twice as many pages for each I/O in a LOB write operation. The following table shows the number of pages that DB2 can write for each I/O operation for a LOB write.

Table 3. The number of pages that DB2 can write for in a single I/O operation for LOB writes
Page Size Number of buffers Number of pages
4 KB BP ≥ 80,000 64
BP < 80,000 32
8 KB BP ≥ 40,000 32
BP < 40,000 16
16 KB BP ≥ 20,000 16
BP < 20,000 8
32 KB BP ≥ 10,000 8
BP < 10,000 4