Mirroring and its effect on the logical log

The logical log is write intensive. If the dbspace that contains the logical-log files is mirrored, you encounter a slight double-write performance penalty. However, you can adjust the rate at which logging generates I/O requests to a certain extent by choosing an appropriate log buffer size and logging mode.

For details on the slight double-write performance penalty, see Consider mirroring for critical data components.

With unbuffered and ANSI-compliant logging, the database server requests a flush of the log buffer to disk for every committed transaction (two when the dbspace is mirrored). Buffered logging generates far fewer I/O requests than unbuffered or ANSI-compliant logging.

With buffered logging, the log buffer is written to disk only when it fills and all the transactions that it contains are completed. You can reduce the frequency of logical-log I/O even more if you increase the size of your logical-log buffers. However, buffered logging leaves transactions in any partially filled buffers vulnerable to loss in the event of a system failure.

Although database consistency is guaranteed under buffered logging, specific transactions are not guaranteed against a failure. The larger the logical-log buffers, the more transactions you might need to reenter when service is restored after a failure.

Unlike the physical log, you cannot specify an alternative dbspace for logical-log files in your initial database server configuration. Instead, use the onparams utility first to add logical-log files to an alternative dbspace and then drop logical-log files from the root dbspace.


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