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11.3. 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, so the operation needs to be as fast as possible. What is worse, writing WAL buffers may also force 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 postgresql.conf WAL_BUFFERS parameter. The default number of WAL buffers is 8. Increasing this value will correspondingly increase 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 undo records are no longer needed and can be recycled or removed. (When WAL-based BAR is implemented, the log segments would be archived before being recycled or 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. (If that happens, the entire database system will be delayed by the creation operation, so it's better if the files can be created in the checkpoint maker, which is not on anyone's critical path.) By default a new 16MB segment file is created only if more than 75% of the current segment has been used. This is inadequate if the system generates more than 4MB of log output between checkpoints. One can instruct the server to pre-create up to 64 log segments at checkpoint time by modifying the WAL_FILES configuration parameter.

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.

Reducing CHECKPOINT_SEGMENTS and/or CHECKPOINT_TIMEOUT causes checkpoints to be done more often. This allows faster after-crash recovery (since less work will need to be redone). However, one must balance this against the increased cost of flushing dirty data pages more often. 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 a smaller checkpoint interval increases the volume of output to the log, partially negating the goal of using a smaller interval, and in any case causing more disk I/O.

The number of 16MB segment files will always be at least WAL_FILES + 1, and will normally not exceed WAL_FILES + MAX(WAL_FILES, CHECKPOINT_SEGMENTS) + 1. This may be used to estimate space requirements for WAL. Ordinarily, when an old log segment files are no longer needed, they are recycled (renamed to become the next sequential future segments). If, due to a short-term peak of log output rate, there are more than WAL_FILES + MAX(WAL_FILES, CHECKPOINT_SEGMENTS) + 1 segment files, then unneeded segment files will be deleted instead of recycled until the system gets back under this limit. (If this happens on a regular basis, WAL_FILES should be increased to avoid it. Deleting log segments that will only have to be created again later is expensive and pointless.)

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. Good values for these parameters are not yet clear; experimentation is encouraged.

The WAL_SYNC_METHOD parameter determines how PostgreSQL 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 nonzero value will result in each LogInsert and LogFlush WAL call being logged to standard error. At present, it makes no difference what the nonzero value is. This option may be replaced by a more general mechanism in the future.

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