Defining, Generating, and Punching a DMCL › Journal Buffers and Journal Files › Sizing Journal Buffers

Sizing Journal Buffers

What the Journal Buffer Defines

The definition of a journal buffer defines how many pages it contains and how large the pages should be.

Buffer Page Size

The journal buffer page size determines the block size for the disk or tape journal files specified for the DMCL. Use the following criteria to choose a size for the journal buffer pages:

• If possible, the page size should be at least twice the size of the longest database record occurrence
• For VSAM disk journals, the buffer page size must be 8 bytes larger than the size of the control interval
• The page size should approximate an optimal page size for the device type in which non-VSAM disk journal files reside
• For tape journal files, the buffer page size should be as large as possible

Note: For more information about valid ranges for each operating system, see the JOURNAL BUFFER statement in Chapter 7, “Physical Database DDL Statements”.

Number of Buffer Pages

The higher the number of buffer pages, the more likely that a journal block will be found in memory eliminating the need for a disk access. Since a central version reads journal blocks primarily during rollback operations, increasing the number of journal buffer pages reduces the number of I/Os and the amount of time needed to roll out database changes.

You should minimally allocate five journal buffer pages. If you have the storage, increase this number significantly in a volatile system in which rollbacks occur frequently.