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 |
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 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.