Postgres-XC 1.1 Documentation
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# 17.5. Write Ahead Log

See also Section 28.4 for details on WAL and checkpoint tuning.

## 17.5.1. Settings

`wal_level` (`enum`)

  `wal_level` determines how much information is written to the WAL. The default value is `minimal`, which writes only the information needed to recover from a crash or immediate shutdown. `archive` adds logging required for WAL archiving, and `hot_standby` further adds information required to run read-only queries on a standby server. This parameter can only be set at server start.

  In `minimal` level, WAL-logging of some bulk operations can be safely skipped, which can make those operations much faster (see Section 14.4.7). Operations in which this optimization can be applied include:

  `CREATE TABLE AS`
  `CREATE INDEX`
  `CLUSTER`
  `COPY` into tables that were created or truncated in the same transaction

  But minimal WAL does not contain enough information to reconstruct the data from a base backup and the WAL logs, so either `archive` or `hot_standby` level must be used to enable WAL archiving (archive_mode) and streaming replication.

  In `hot_standby` level, the same information is logged as with `archive`, plus information needed to reconstruct the status of running transactions from the WAL. To enable read-only queries on a standby server, `wal_level` must be set to `hot_standby` on the primary, and hot_standby must be enabled in the standby. It is thought that there is little measurable difference in performance between using `hot_standby` and `archive` levels, so feedback is welcome if any production impacts are noticeable.

`fsync` (`boolean`)

  If this parameter is on, coordinator or datanode will try to make sure that updates are physically written to disk, by issuing `fsync()` system calls or various equivalent methods (see wal_sync_method). This ensures that the database cluster can recover to a consistent state after an operating system or hardware crash.

  While turning off `fsync` is often a performance benefit, this can result in unrecoverable data corruption in the event of a power failure or system crash. Thus it is only advisable to turn off `fsync` if you can easily recreate your entire database from external data.

  Examples of safe circumstances for turning off `fsync` include the initial loading of a new database cluster from a backup file, using a database cluster for processing a batch of data after which the database will be thrown away and recreated, or for a read-only database clone which gets recreated frequently and is not used for failover. High quality hardware alone is not a sufficient justification for turning off `fsync`.

  In many situations, turning off synchronous_commit for noncritical transactions can provide much of the potential performance benefit of turning off `fsync`, without the attendant risks of data corruption.

  `fsync` can only be set in the `postgresql.conf` file or on the server command line. If you turn this parameter off, also consider turning off full_page_writes.

`synchronous_commit` (`enum`)

  Specifies whether transaction commit will wait for WAL records to be written to disk before the command returns a "success" indication to the client. Valid values are `on`, `remote_write`, `local`, and `off`. The default, and safe, setting is `on`. When `off`, there can be a delay between when success is reported to the client and when the transaction is really guaranteed to be safe against a server crash. (The maximum delay is three times wal_writer_delay.) Unlike fsync, setting this parameter to `off` does not create any risk of database inconsistency: an operating system or database crash might result in some recent allegedly-committed transactions being lost, but the database state will be just the same as if those transactions had been aborted cleanly. So, turning `synchronous_commit` off can be a useful alternative when performance is more important than exact certainty about the durability of a transaction. For more discussion see Section 28.3.

  If synchronous_standby_names is set, this parameter also controls whether or not transaction commits will wait for the transaction's WAL records to be replicated to the standby server. When set to `on`, commits will wait until a reply from the current synchronous standby indicates it has received the commit record of the transaction and flushed it to disk. This ensures the transaction will not be lost unless both primary and standby suffer corruption of their database storage. When set to `remote_write`, commits will wait until a reply from the current synchronous standby indicates it has received the commit record of the transaction and written it out to the standby's operating system, but the data has not necessarily reached stable storage on the standby. This setting is sufficient to ensure data preservation even if the standby instance of coordinator or datanode were to crash, but not if the standby suffers an operating-system-level crash.

  When synchronous replication is in use, it will normally be sensible either to wait for both local flush to disk and replication of WAL records, or to allow the transaction to commit asynchronously. However, the setting `local` is available for transactions that wish to wait for local flush to disk, but not synchronous replication. If `synchronous_standby_names` is not set, the settings `on`, `remote_write` and `local` all provide the same synchronization level: transaction commits only wait for local flush to disk.

  This parameter can be changed at any time; the behavior for any one transaction is determined by the setting in effect when it commits. It is therefore possible, and useful, to have some transactions commit synchronously and others asynchronously. For example, to make a single multistatement transaction commit asynchronously when the default is the opposite, issue `SET LOCAL synchronous_commit TO OFF` within the transaction.

`wal_sync_method` (`enum`)

  Method used for forcing WAL updates out to disk. If `fsync` is off then this setting is irrelevant, since WAL file updates will not be forced out at all. Possible values are:

  - `open_datasync` (write WAL files with `open()` option `O_DSYNC`)

  - `fdatasync` (call `fdatasync()` at each commit)

  - `fsync` (call `fsync()` at each commit)

  - `fsync_writethrough` (call `fsync()` at each commit, forcing write-through of any disk write cache)

  - `open_sync` (write WAL files with `open()` option `O_SYNC`)

  The `open_`* options also use `O_DIRECT` if available. Not all of these choices are available on all platforms. The default is the first method in the above list that is supported by the platform, except that `fdatasync` is the default on Linux. The default is not necessarily ideal; it might be necessary to change this setting or other aspects of your system configuration in order to create a crash-safe configuration or achieve optimal performance. These aspects are discussed in Section 28.1. This parameter can only be set in the `postgresql.conf` file or on the server command line.

`full_page_writes` (`boolean`)

  When this parameter is on, the coordinator or the datanode writes the entire content of each disk page to WAL during the first modification of that page after a checkpoint. This is needed because a page write that is in process during an operating system crash might be only partially completed, leading to an on-disk page that contains a mix of old and new data. The row-level change data normally stored in WAL will not be enough to completely restore such a page during post-crash recovery. Storing the full page image guarantees that the page can be correctly restored, but at the price of increasing the amount of data that must be written to WAL. (Because WAL replay always starts from a checkpoint, it is sufficient to do this during the first change of each page after a checkpoint. Therefore, one way to reduce the cost of full-page writes is to increase the checkpoint interval parameters.)

  Turning this parameter off speeds normal operation, but might lead to either unrecoverable data corruption, or silent data corruption, after a system failure. The risks are similar to turning off `fsync`, though smaller, and it should be turned off only based on the same circumstances recommended for that parameter.

  Turning off this parameter does not affect use of WAL archiving for point-in-time recovery (PITR) (see Section 23.3).

  This parameter can only be set in the `postgresql.conf` file or on the server command line. The default is `on`.

`wal_buffers` (`integer`)

  The amount of shared memory used for WAL data that has not yet been written to disk. The default setting of -1 selects a size equal to 1/32nd (about 3%) of shared_buffers, but not less than `64kB` nor more than the size of one WAL segment, typically `16MB`. This value can be set manually if the automatic choice is too large or too small, but any positive value less than `32kB` will be treated as `32kB`. This parameter can only be set at server start.

  The contents of the WAL buffers are written out to disk at every transaction commit, so extremely large values are unlikely to provide a significant benefit. However, setting this value to at least a few megabytes can improve write performance on a busy server where many clients are committing at once. The auto-tuning selected by the default setting of -1 should give reasonable results in most cases.

  Increasing this parameter might cause coordinator or datanode to request more System V shared memory than your operating system's default configuration allows. See Section 16.4.1 for information on how to adjust those parameters, if necessary.

`wal_writer_delay` (`integer`)

  Specifies the delay between activity rounds for the WAL writer. In each round the writer will flush WAL to disk. It then sleeps for `wal_writer_delay` milliseconds, and repeats. The default value is 200 milliseconds (`200ms`). Note that on many systems, the effective resolution of sleep delays is 10 milliseconds; setting `wal_writer_delay` to a value that is not a multiple of 10 might have the same results as setting it to the next higher multiple of 10. This parameter can only be set in the `postgresql.conf` file or on the server command line.

`commit_delay` (`integer`)

  When the commit data for a transaction is flushed to disk, any additional commits ready at that time are also flushed out. `commit_delay` adds a time delay, set in microseconds, before a transaction attempts to flush the WAL buffer out to disk. A nonzero delay can allow more transactions to be committed with only one flush operation, if system load is high enough that additional transactions become ready to commit within the given interval. But the delay is just wasted if no other transactions become ready to commit. Therefore, the delay is only performed if at least `commit_siblings` other transactions are active at the instant that a server process has written its commit record. The default `commit_delay` is zero (no delay). Since all pending commit data will be written at every flush regardless of this setting, it is rare that adding delay by increasing this parameter will actually improve performance.

`commit_siblings` (`integer`)

  Minimum number of concurrent open transactions to require before performing the `commit_delay` delay. A larger value makes it more probable that at least one other transaction will become ready to commit during the delay interval. The default is five transactions.

## 17.5.2. Checkpoints

`checkpoint_segments` (`integer`)

  Maximum number of log file segments between automatic WAL checkpoints (each segment is normally 16 megabytes). The default is three segments. Increasing this parameter can increase the amount of time needed for crash recovery. This parameter can only be set in the `postgresql.conf` file or on the server command line.

`checkpoint_timeout` (`integer`)

  Maximum time between automatic WAL checkpoints, in seconds. The default is five minutes (`5min`). Increasing this parameter can increase the amount of time needed for crash recovery. This parameter can only be set in the `postgresql.conf` file or on the server command line.

`checkpoint_completion_target` (`floating point`)

  Specifies the target of checkpoint completion, as a fraction of total time between checkpoints. The default is 0.5. This parameter can only be set in the `postgresql.conf` file or on the server command line.

`checkpoint_warning` (`integer`)

  Write a message to the server log if checkpoints caused by the filling of checkpoint segment files happen closer together than this many seconds (which suggests that `checkpoint_segments` ought to be raised). The default is 30 seconds (`30s`). Zero disables the warning. This parameter can only be set in the `postgresql.conf` file or on the server command line.

## 17.5.3. Archiving

`archive_mode` (`boolean`)

  When `archive_mode` is enabled, completed WAL segments are sent to archive storage by setting archive_command. `archive_mode` and `archive_command` are separate variables so that `archive_command` can be changed without leaving archiving mode. This parameter can only be set at server start. `archive_mode` cannot be enabled when `wal_level` is set to `minimal`.

`archive_command` (`string`)

  The shell command to execute to archive a completed WAL file segment. Any `%p` in the string is replaced by the path name of the file to archive, and any `%f` is replaced by only the file name. (The path name is relative to the working directory of the server, i.e., the cluster's data directory.) Use `%%` to embed an actual `%` character in the command. It is important for the command to return a zero exit status only if it succeeds. For more information see Section 23.3.1.

  This parameter can only be set in the `postgresql.conf` file or on the server command line. It is ignored unless `archive_mode` was enabled at server start. If `archive_command` is an empty string (the default) while `archive_mode` is enabled, WAL archiving is temporarily disabled, but the server continues to accumulate WAL segment files in the expectation that a command will soon be provided. Setting `archive_command` to a command that does nothing but return true, e.g. `/bin/true` (`REM` on Windows), effectively disables archiving, but also breaks the chain of WAL files needed for archive recovery, so it should only be used in unusual circumstances.

`archive_timeout` (`integer`)

  The archive_command is only invoked for completed WAL segments. Hence, if your server generates little WAL traffic (or has slack periods where it does so), there could be a long delay between the completion of a transaction and its safe recording in archive storage. To limit how old unarchived data can be, you can set `archive_timeout` to force the server to switch to a new WAL segment file periodically. When this parameter is greater than zero, the server will switch to a new segment file whenever this many seconds have elapsed since the last segment file switch, and there has been any database activity, including a single checkpoint. (Increasing `checkpoint_timeout` will reduce unnecessary checkpoints on an idle system.) Note that archived files that are closed early due to a forced switch are still the same length as completely full files. Therefore, it is unwise to use a very short `archive_timeout` — it will bloat your archive storage. `archive_timeout` settings of a minute or so are usually reasonable. You should consider using streaming replication, instead of archiving, if you want data to be copied off the master server more quickly than that. This parameter can only be set in the `postgresql.conf` file or on the server command line.

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