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Specifying Shared Memory for Log Buffers

The size that you specify for both the logical-log and physical-log buffers affects logging I/O activity as well as the amount of shared memory required by the database server. The sizes of the logical- and physical-log buffers in shared memory determines how quickly the logs fill and therefore how often they are flushed to disk.

To monitor physical and logical log buffer size and activity, use onstat -l. For the physical log, the output of onstat -l should show that the pages to I/O ratio is approximately 75 percent of the bufsize column.

The Logical-Log Buffer Size

For each of the three buffers that hold the logical-log records until they are flushed to the logical-log file on disk, set the LOGBUFF configuration parameter to specify a specific amount of memory. The minimum size of a logical-log buffer is two times the page size.

The amount of logical-log buffer space you need depends on whether logging is buffered. If logging is not buffered, buffer-flushing intervals depend on the size of the transactions and not on how much buffer space is available. If most transactions are smaller than the buffer page size, the ratio of pages to I/O to bufsize might always be low.

The Physical-Log Buffer Size

For each of the two buffers that hold data pages that are about to be modified before they are flushed to the physical log on disk, set the PHYSBUFF configuration parameter to specify a specific amount of memory. The minimum size of a physical-log buffer is one page, as determined by the page size you specify with the PAGESIZE configuration parameter. The recommended size is the equivalent of 16 pages. For the default page size of 4 kilobytes, this amount is 64 kilobytes for each physical log buffer.

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