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There are severalWAL-related configuration parameters that affect database performance. This section explains their use. ConsultChapter 18 for general information about setting server configuration parameters.

Checkpoints are points in the sequence of transactions at which it is guaranteed that the heap and index data files have been updated with all information written before that checkpoint. At checkpoint time, all dirty data pages are flushed to disk and a special checkpoint record is written to the log file. (The change records were previously flushed to theWAL files.) In the event of a crash, the crash recovery procedure looks at the latest checkpoint record to determine the point in the log (known as the redo record) from which it should start the REDO operation. Any changes made to data files before that point are guaranteed to be already on disk. Hence, after a checkpoint, log segments preceding the one containing the redo record are no longer needed and can be recycled or removed. (WhenWAL archiving is being done, the log segments must be archived before being recycled or removed.)

The checkpoint requirement of flushing all dirty data pages to disk can cause a significant I/O load. For this reason, checkpoint activity is throttled so that I/O begins at checkpoint start and completes before the next checkpoint is due to start; this minimizes performance degradation during checkpoints.

The server's checkpointer process automatically performs a checkpoint every so often. A checkpoint is begun everycheckpoint_timeout seconds, or ifmax_wal_size is about to be exceeded, whichever comes first. The default settings are 5 minutes and 1 GB, respectively. If no WAL has been written since the previous checkpoint, new checkpoints will be skipped even ifcheckpoint_timeout has passed. (If WAL archiving is being used and you want to put a lower limit on how often files are archived in order to bound potential data loss, you should adjust thearchive_timeout parameter rather than the checkpoint parameters.) It is also possible to force a checkpoint by using the SQL commandCHECKPOINT.

Reducingcheckpoint_timeout and/ormax_wal_size causes checkpoints to occur 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. Iffull_page_writes is set (as is the default), there is another factor to consider. To ensure data page consistency, the first modification of a data page after each checkpoint results in logging the entire page content. In that case, a smaller checkpoint interval increases the volume of output to the WAL log, partially negating the goal of using a smaller interval, and in any case causing more disk I/O.

Checkpoints are fairly expensive, first because they require writing out all currently dirty buffers, and second because they result in extra subsequent WAL traffic as discussed above. It is therefore wise to set the checkpointing parameters high enough so that checkpoints don't happen too often. As a simple sanity check on your checkpointing parameters, you can set thecheckpoint_warning parameter. If checkpoints happen closer together thancheckpoint_warning seconds, a message will be output to the server log recommending increasingmax_wal_size. Occasional appearance of such a message is not cause for alarm, but if it appears often then the checkpoint control parameters should be increased. Bulk operations such as largeCOPY transfers might cause a number of such warnings to appear if you have not setmax_wal_size high enough.

To avoid flooding the I/O system with a burst of page writes, writing dirty buffers during a checkpoint is spread over a period of time. That period is controlled bycheckpoint_completion_target, which is given as a fraction of the checkpoint interval. The I/O rate is adjusted so that the checkpoint finishes when the given fraction ofcheckpoint_timeout seconds have elapsed, or beforemax_wal_size is exceeded, whichever is sooner. With the default value of 0.5,PostgreSQL can be expected to complete each checkpoint in about half the time before the next checkpoint starts. On a system that's very close to maximum I/O throughput during normal operation, you might want to increasecheckpoint_completion_target to reduce the I/O load from checkpoints. The disadvantage of this is that prolonging checkpoints affects recovery time, because more WAL segments will need to be kept around for possible use in recovery. Althoughcheckpoint_completion_target can be set as high as 1.0, it is best to keep it less than that (perhaps 0.9 at most) since checkpoints include some other activities besides writing dirty buffers. A setting of 1.0 is quite likely to result in checkpoints not being completed on time, which would result in performance loss due to unexpected variation in the number of WAL segments needed.

The number of WAL segment files inpg_xlog directory depends onmin_wal_size,max_wal_size and the amount of WAL generated in previous checkpoint cycles. When old log segment files are no longer needed, they are removed or recycled (that is, renamed to become future segments in the numbered sequence). If, due to a short-term peak of log output rate,max_wal_size is exceeded, the unneeded segment files will be removed until the system gets back under this limit. Below that limit, the system recycles enough WAL files to cover the estimated need until the next checkpoint, and removes the rest. The estimate is based on a moving average of the number of WAL files used in previous checkpoint cycles. The moving average is increased immediately if the actual usage exceeds the estimate, so it accommodates peak usage rather than average usage to some extent.min_wal_size puts a minimum on the amount of WAL files recycled for future usage; that much WAL is always recycled for future use, even if the system is idle and the WAL usage estimate suggests that little WAL is needed.

Independently ofmax_wal_size,wal_keep_segments + 1 most recent WAL files are kept at all times. Also, if WAL archiving is used, old segments can not be removed or recycled until they are archived. If WAL archiving cannot keep up with the pace that WAL is generated, or ifarchive_command fails repeatedly, old WAL files will accumulate inpg_xlog until the situation is resolved. A slow or failed standby server that uses a replication slot will have the same effect (seeSection 25.2.6).

In archive recovery or standby mode, the server periodically performsrestartpoints, which are similar to checkpoints in normal operation: the server forces all its state to disk, updates thepg_control file to indicate that the already-processed WAL data need not be scanned again, and then recycles any old log segment files in thepg_xlog directory. Restartpoints can't be performed more frequently than checkpoints in the master because restartpoints can only be performed at checkpoint records. A restartpoint is triggered when a checkpoint record is reached if at leastcheckpoint_timeout seconds have passed since the last restartpoint, or if WAL size is about to exceedmax_wal_size. However, because of limitations on when a restartpoint can be performed,max_wal_size is often exceeded during recovery, by up to one checkpoint cycle's worth of WAL. (max_wal_size is never a hard limit anyway, so you should always leave plenty of headroom to avoid running out of disk space.)

There are two commonly used internalWAL functions:XLogInsertRecord andXLogFlush.XLogInsertRecord is used to place a new record into theWAL buffers in shared memory. If there is no space for the new record,XLogInsertRecord will have to write (move to kernel cache) a few filledWAL buffers. This is undesirable becauseXLogInsertRecord is used on every database low level modification (for example, row 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, writingWAL buffers might also force the creation of a new log segment, which takes even more time. Normally,WAL buffers should be written and flushed by anXLogFlush 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,XLogFlush requests might not occur often enough to preventXLogInsertRecord from having to do writes. On such systems one should increase the number ofWAL buffers by modifying thewal_buffers parameter. Whenfull_page_writes is set and the system is very busy, settingwal_buffers higher will help smooth response times during the period immediately following each checkpoint.

Thecommit_delay parameter defines for how many microseconds a group commit leader process will sleep after acquiring a lock withinXLogFlush, while group commit followers queue up behind the leader. This delay allows other server processes to add their commit records to the WAL buffers so that all of them will be flushed by the leader's eventual sync operation. No sleep will occur iffsync is not enabled, or if fewer thancommit_siblings other sessions are currently in active transactions; this avoids sleeping when it's unlikely that any other session will commit soon. Note that on some platforms, the resolution of a sleep request is ten milliseconds, so that any nonzerocommit_delay setting between 1 and 10000 microseconds would have the same effect. Note also that on some platforms, sleep operations may take slightly longer than requested by the parameter.

Since the purpose ofcommit_delay is to allow the cost of each flush operation to be amortized across concurrently committing transactions (potentially at the expense of transaction latency), it is necessary to quantify that cost before the setting can be chosen intelligently. The higher that cost is, the more effectivecommit_delay is expected to be in increasing transaction throughput, up to a point. Thepg_test_fsync program can be used to measure the average time in microseconds that a single WAL flush operation takes. A value of half of the average time the program reports it takes to flush after a single 8kB write operation is often the most effective setting forcommit_delay, so this value is recommended as the starting point to use when optimizing for a particular workload. While tuningcommit_delay is particularly useful when the WAL log is stored on high-latency rotating disks, benefits can be significant even on storage media with very fast sync times, such as solid-state drives or RAID arrays with a battery-backed write cache; but this should definitely be tested against a representative workload. Higher values ofcommit_siblings should be used in such cases, whereas smallercommit_siblings values are often helpful on higher latency media. Note that it is quite possible that a setting ofcommit_delay that is too high can increase transaction latency by so much that total transaction throughput suffers.

Whencommit_delay is set to zero (the default), it is still possible for a form of group commit to occur, but each group will consist only of sessions that reach the point where they need to flush their commit records during the window in which the previous flush operation (if any) is occurring. At higher client counts a“gangway effect” tends to occur, so that the effects of group commit become significant even whencommit_delay is zero, and thus explicitly settingcommit_delay tends to help less. Settingcommit_delay can only help when (1) there are some concurrently committing transactions, and (2) throughput is limited to some degree by commit rate; but with high rotational latency this setting can be effective in increasing transaction throughput with as few as two clients (that is, a single committing client with one sibling transaction).

Thewal_sync_method parameter determines howPostgreSQL will ask the kernel to forceWAL updates out to disk. All the options should be the same in terms of reliability, with the exception offsync_writethrough, which can sometimes force a flush of the disk cache even when other options do not do so. However, it's quite platform-specific which one will be the fastest. You can test the speeds of different options using thepg_test_fsync program. Note that this parameter is irrelevant iffsync has been turned off.

Enabling thewal_debug configuration parameter (provided thatPostgreSQL has been compiled with support for it) will result in eachXLogInsertRecord andXLogFlushWAL call being logged to the server log. This option might be replaced by a more general mechanism in the future.