For additional information on tuning these settings, see Section 30.4.
wal_level (enum)
      
              wal_level determines how much information is written to
        the WAL. The default value is replica, which writes enough
        data to support WAL archiving and replication, including running
        read-only queries on a standby server. minimal removes all
        logging except the information required to recover from a crash or
        immediate shutdown.  Finally,
        logical adds information necessary to support logical
        decoding.  Each level includes the information logged at all lower
        levels.  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 | 
| COPYinto 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 replica or
        higher must be used to enable WAL archiving
        (archive_mode) and streaming replication.
       
        In logical level, the same information is logged as
        with replica, plus information needed to allow
        extracting logical change sets from the WAL. Using a level of
        logical will increase the WAL volume, particularly if many
        tables are configured for REPLICA IDENTITY FULL and
        many UPDATE and DELETE statements are
        executed.
       
        In releases prior to 9.6, this parameter also allowed the
        values archive and hot_standby.
        These are still accepted but mapped to replica.
       
fsync (boolean)
      
              If this parameter is on, the PostgreSQL server
        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.
       
        For reliable recovery when changing fsync
        off to on, it is necessary to force all modified buffers in the
        kernel to durable storage.  This can be done while the cluster
        is shutdown or while fsync is on by running initdb
        --sync-only, running sync, unmounting the
        file system, or rebooting the server.
       
        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_apply, 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 30.3.
       
        If synchronous_standby_names is non-empty, this
        parameter also controls whether or not transaction commits will wait
        for their WAL records to be replicated to the standby server(s).
        When set to on, commits will wait until replies
        from the current synchronous standby(s) indicate they have received
        the commit record of the transaction and flushed it to disk.  This
        ensures the transaction will not be lost unless both the primary and
        all synchronous standbys suffer corruption of their database storage.
        When set to remote_apply, commits will wait until replies
        from the current synchronous standby(s) indicate they have received the
        commit record of the transaction and applied it, so that it has become
        visible to queries on the standby(s).
        When set to remote_write, commits will wait until replies
        from the current synchronous standby(s) indicate they have
        received the commit record of the transaction and written it out to
        their operating system. This setting is sufficient to
        ensure data preservation even if a standby instance of
        PostgreSQL were to crash, but not if the standby
        suffers an operating-system-level crash, since the data has not
        necessarily reached stable storage on the standby.
        Finally, the setting local causes commits to wait for
        local flush to disk, but not for replication.  This is not usually
        desirable when synchronous replication is in use, but is provided for
        completeness.
       
        If synchronous_standby_names is empty, the settings
        on, remote_apply, 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 30.1.
        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 PostgreSQL server 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 25.3).
        This parameter can only be set in the postgresql.conf
        file or on the server command line.
        The default is on.
       
wal_log_hints (boolean)
      
              When this parameter is on, the PostgreSQL
        server writes the entire content of each disk page to WAL during the
        first modification of that page after a checkpoint, even for
        non-critical modifications of so-called hint bits.
       
If data checksums are enabled, hint bit updates are always WAL-logged and this setting is ignored. You can use this setting to test how much extra WAL-logging would occur if your database had data checksums enabled.
        This parameter can only be set at server start. The default value is off.
       
wal_compression (boolean)
      
              When this parameter is on, the PostgreSQL
        server compresses a full page image written to WAL when
        full_page_writes is on or during a base backup.
        A compressed page image will be decompressed during WAL replay.
        The default value is off.
        Only superusers can change this setting.
       
Turning this parameter on can reduce the WAL volume without increasing the risk of unrecoverable data corruption, but at the cost of some extra CPU spent on the compression during WAL logging and on the decompression during WAL replay.
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.
wal_writer_delay (integer)
      
              Specifies how often the WAL writer flushes WAL. After flushing WAL it
        sleeps for wal_writer_delay milliseconds, unless woken up
        by an asynchronously committing transaction. If the last flush
        happened less than wal_writer_delay milliseconds ago and
        less than wal_writer_flush_after bytes of WAL have been
        produced since, then WAL is only written to the operating system, not
        flushed to disk.
        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.
       
wal_writer_flush_after (integer)
      
              Specifies how often the WAL writer flushes WAL. If the last flush
        happened less than wal_writer_delay milliseconds ago and
        less than wal_writer_flush_after bytes of WAL have been
        produced since, then WAL is only written to the operating system, not
        flushed to disk.  If wal_writer_flush_after is set
        to 0 then WAL data is flushed immediately.  The default is
        1MB. This parameter can only be set in the
        postgresql.conf file or on the server command line.
       
commit_delay (integer)
      
              commit_delay adds a time delay, measured in
        microseconds, before a WAL flush is initiated.  This can improve
        group commit throughput by allowing a larger number of transactions
        to commit via a single WAL flush, if system load is high enough
        that additional transactions become ready to commit within the
        given interval.  However, it also increases latency by up to
        commit_delay microseconds for each WAL
        flush.  Because the delay is just wasted if no other transactions
        become ready to commit, a delay is only performed if at least
        commit_siblings other transactions are active
        when a flush is about to be initiated.  Also, no delays are
        performed if fsync is disabled.
        The default commit_delay is zero (no delay).
        Only superusers can change this setting.
       
        In PostgreSQL releases prior to 9.3,
        commit_delay behaved differently and was much
        less effective: it affected only commits, rather than all WAL flushes,
        and waited for the entire configured delay even if the WAL flush
        was completed sooner.  Beginning in PostgreSQL 9.3,
        the first process that becomes ready to flush waits for the configured
        interval, while subsequent processes wait only until the leader
        completes the flush operation.
       
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.
       
checkpoint_timeout (integer)
      
              Maximum time between automatic WAL checkpoints, in seconds.
        The valid range is between 30 seconds and one day.
        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_flush_after (integer)
      
              Whenever more than checkpoint_flush_after bytes
        have been written while performing a checkpoint, attempt to force the
        OS to issue these writes to the underlying storage.  Doing so will
        limit the amount of dirty data in the kernel's page cache, reducing
        the likelihood of stalls when an fsync is issued at the end of the
        checkpoint, or when the OS writes data back in larger batches in the
        background.  Often that will result in greatly reduced transaction
        latency, but there also are some cases, especially with workloads
        that are bigger than shared_buffers, but smaller
        than the OS's page cache, where performance might degrade.  This
        setting may have no effect on some platforms.  The valid range is
        between 0, which disables forced writeback,
        and 2MB.  The default is 256kB on
        Linux, 0 elsewhere.  (If BLCKSZ is not
        8kB, the default and maximum values scale proportionally to it.)
        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
        max_wal_size ought to be raised).  The default is
        30 seconds (30s).  Zero disables the warning.
        No warnings will be generated if checkpoint_timeout
        is less than checkpoint_warning.
        This parameter can only be set in the postgresql.conf
        file or on the server command line.
       
max_wal_size (integer)
      
              Maximum size to let the WAL grow to between automatic WAL
        checkpoints. This is a soft limit; WAL size can exceed
        max_wal_size under special circumstances, like
        under heavy load, a failing archive_command, or a high
        wal_keep_segments setting. The default is 1 GB.
        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.
       
min_wal_size (integer)
      
              As long as WAL disk usage stays below this setting, old WAL files are
        always recycled for future use at a checkpoint, rather than removed.
        This can be used to ensure that enough WAL space is reserved to
        handle spikes in WAL usage, for example when running large batch
        jobs. The default is 80 MB.
        This parameter can only be set in the postgresql.conf
        file or on the server command line.
       
archive_mode (enum)
      
              When archive_mode is enabled, completed WAL segments
        are sent to archive storage by setting
        archive_command. In addition to off,
        to disable, there are two modes: on, and
        always. During normal operation, there is no
        difference between the two modes, but when set to always
        the WAL archiver is enabled also during archive recovery or standby
        mode. In always mode, all files restored from the archive
        or streamed with streaming replication will be archived (again). See
        Section 26.2.9 for details.
       
        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 local 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 25.3.1.
       
        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 (checkpoints are skipped if there is
        no database activity).  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.