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9.3. WAL Configuration

There are several WAL-related parameters that affect database performance. This section explains their use. Consult Section 3.4 for details about setting configuration parameters.

There are two commonly used WAL functions: LogInsert and LogFlush. LogInsert is used to place a new record into the WAL buffers in shared memory. If there is no space for the new record, LogInsert will have to write (move to kernel cache) a few filled WAL buffers. This is undesirable because LogInsert is used on every database low level modification (for example, tuple insertion) at a time when an exclusive lock is held on affected data pages and the operation is supposed to be as fast as possible; what is worse, writing WAL buffers may also cause the creation of a new log segment, which takes even more time. Normally, WAL buffers should be written and flushed by a LogFlush request, which is made, for the most part, at transaction commit time to ensure that transaction records are flushed to permanent storage. On systems with high log output, LogFlush requests may not occur often enough to prevent WAL buffers being written by LogInsert. On such systems one should increase the number of WAL buffers by modifying the WAL_BUFFERS parameter. The default number of WAL buffers is 8. Increasing this value will have an impact on shared memory usage.

Checkpoints are points in the sequence of transactions at which it is guaranteed that the data files have been updated with all information logged before the checkpoint. At checkpoint time, all dirty data pages are flushed to disk and a special checkpoint record is written to the log file. As result, in the event of a crash, the recoverer knows from what record in the log (known as the redo record) it should start the REDO operation, since any changes made to data files before that record are already on disk. After a checkpoint has been made, any log segments written before the redo record are removed, so checkpoints are used to free disk space in the WAL directory. (When WAL-based BAR is implemented, the log segments can be archived instead of just being removed.) The checkpoint maker is also able to create a few log segments for future use, so as to avoid the need for LogInsert or LogFlush to spend time in creating them.

The WAL log is held on the disk as a set of 16 MB files called segments. By default a new segment is created only if more than 75% of the current segment is used. One can instruct the server to pre-create up to 64 log segments at checkpoint time by modifying the WAL_FILES configuration parameter.

For faster after-crash recovery, it would be better to create checkpoints more often. However, one should balance this against the cost of flushing dirty data pages; in addition, to ensure data page consistency, the first modification of a data page after each checkpoint results in logging the entire page content, thus increasing output to log and the log's size.

The postmaster spawns a special backend process every so often to create the next checkpoint. A checkpoint is created every CHECKPOINT_SEGMENTS log segments, or every CHECKPOINT_TIMEOUT seconds, whichever comes first. The default settings are 3 segments and 300 seconds respectively. It is also possible to force a checkpoint by using the SQL command CHECKPOINT.

The COMMIT_DELAY parameter defines for how many microseconds the backend will sleep after writing a commit record to the log with LogInsert but before performing a LogFlush. This delay allows other backends to add their commit records to the log so as to have all of them flushed with a single log sync. No sleep will occur if fsync is not enabled or if fewer than COMMIT_SIBLINGS other backends are not currently in active transactions; this avoids sleeping when it's unlikely that any other backend will commit soon. Note that on most platforms, the resolution of a sleep request is ten milliseconds, so that any nonzero COMMIT_DELAY setting between 1 and 10000 microseconds will have the same effect.

The WAL_SYNC_METHOD parameter determines how Postgres will ask the kernel to force WAL updates out to disk. All the options should be the same as far as reliability goes, but it's quite platform-specific which one will be the fastest. Note that this parameter is irrelevant if FSYNC has been turned off.

Setting the WAL_DEBUG parameter to any non-zero value will result in each LogInsert and LogFlush WAL call being logged to standard error. At present, it makes no difference what the non-zero value is. This option may be replaced by a more general mechanism in the future.