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CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="RUNTIME-CONFIG-WAL"
>18.5. Write Ahead Log</A
></H1
><P
> See also <A
HREF="wal-configuration.html"
>Section 28.4</A
> for details on WAL
and checkpoint tuning.
</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="RUNTIME-CONFIG-WAL-SETTINGS"
>18.5.1. Settings</A
></H2
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><A
NAME="GUC-FSYNC"
></A
><TT
CLASS="VARNAME"
>fsync</TT
> (<TT
CLASS="TYPE"
>boolean</TT
>)</DT
><DD
><P
> If this parameter is on, the <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> server
will try to make sure that updates are physically written to
disk, by issuing <CODE
CLASS="FUNCTION"
>fsync()</CODE
> system calls or various
equivalent methods (see <A
HREF="runtime-config-wal.html#GUC-WAL-SYNC-METHOD"
>wal_sync_method</A
>).
This ensures that the database cluster can recover to a
consistent state after an operating system or hardware crash.
</P
><P
> However, using <TT
CLASS="VARNAME"
>fsync</TT
> results in a
performance penalty: when a transaction is committed,
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> must wait for the
operating system to flush the write-ahead log to disk. When
<TT
CLASS="VARNAME"
>fsync</TT
> is disabled, the operating system is
allowed to do its best in buffering, ordering, and delaying
writes. This can result in significantly improved performance.
However, if the system crashes, the results of the last few
committed transactions might be lost in part or whole. In the
worst case, unrecoverable data corruption might occur.
(Crashes of the database software itself are <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>not</I
></SPAN
>
a risk factor here. Only an operating-system-level crash
creates a risk of corruption.)
</P
><P
> Due to the risks involved, there is no universally correct
setting for <TT
CLASS="VARNAME"
>fsync</TT
>. Some administrators
always disable <TT
CLASS="VARNAME"
>fsync</TT
>, while others only
turn it off during initial bulk data loads, where there is a clear
restart point if something goes wrong. Others
always leave <TT
CLASS="VARNAME"
>fsync</TT
> enabled. The default is
to enable <TT
CLASS="VARNAME"
>fsync</TT
>, for maximum reliability.
If you trust your operating system, your hardware, and your
utility company (or your battery backup), you can consider
disabling <TT
CLASS="VARNAME"
>fsync</TT
>.
</P
><P
> In many situations, turning off <A
HREF="runtime-config-wal.html#GUC-SYNCHRONOUS-COMMIT"
>synchronous_commit</A
>
for noncritical transactions can provide much of the potential
performance benefit of turning off <TT
CLASS="VARNAME"
>fsync</TT
>, without
the attendant risks of data corruption.
</P
><P
> This parameter can only be set in the <TT
CLASS="FILENAME"
>postgresql.conf</TT
>
file or on the server command line.
If you turn this parameter off, also consider turning off
<A
HREF="runtime-config-wal.html#GUC-FULL-PAGE-WRITES"
>full_page_writes</A
>.
</P
></DD
><DT
><A
NAME="GUC-SYNCHRONOUS-COMMIT"
></A
><TT
CLASS="VARNAME"
>synchronous_commit</TT
> (<TT
CLASS="TYPE"
>boolean</TT
>)</DT
><DD
><P
> Specifies whether transaction commit will wait for WAL records
to be written to disk before the command returns a <SPAN
CLASS="QUOTE"
>"success"</SPAN
>
indication to the client. The default, and safe, setting is
<TT
CLASS="LITERAL"
>on</TT
>. When <TT
CLASS="LITERAL"
>off</TT
>, 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 <A
HREF="runtime-config-wal.html#GUC-WAL-WRITER-DELAY"
>wal_writer_delay</A
>.) Unlike
<A
HREF="runtime-config-wal.html#GUC-FSYNC"
>fsync</A
>, setting this parameter to <TT
CLASS="LITERAL"
>off</TT
>
does not create any risk of database inconsistency: a 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 <TT
CLASS="VARNAME"
>synchronous_commit</TT
> off
can be a useful alternative when performance is more important than
exact certainty about the durability of a transaction. For more
discussion see <A
HREF="wal-async-commit.html"
>Section 28.3</A
>.
</P
><P
> 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 multi-statement transaction commit
asynchronously when the default is the opposite, issue <TT
CLASS="COMMAND"
>SET
LOCAL synchronous_commit TO OFF</TT
> within the transaction.
</P
></DD
><DT
><A
NAME="GUC-WAL-SYNC-METHOD"
></A
><TT
CLASS="VARNAME"
>wal_sync_method</TT
> (<TT
CLASS="TYPE"
>enum</TT
>)</DT
><DD
><P
> Method used for forcing WAL updates out to disk.
If <TT
CLASS="VARNAME"
>fsync</TT
> is off then this setting is irrelevant,
since updates will not be forced out at all.
Possible values are:
</P
><P
></P
><UL
><LI
><P
> <TT
CLASS="LITERAL"
>open_datasync</TT
> (write WAL files with <CODE
CLASS="FUNCTION"
>open()</CODE
> option <TT
CLASS="SYMBOL"
>O_DSYNC</TT
>)
</P
></LI
><LI
><P
> <TT
CLASS="LITERAL"
>fdatasync</TT
> (call <CODE
CLASS="FUNCTION"
>fdatasync()</CODE
> at each commit)
</P
></LI
><LI
><P
> <TT
CLASS="LITERAL"
>fsync</TT
> (call <CODE
CLASS="FUNCTION"
>fsync()</CODE
> at each commit)
</P
></LI
><LI
><P
> <TT
CLASS="LITERAL"
>fsync_writethrough</TT
> (call <CODE
CLASS="FUNCTION"
>fsync()</CODE
> at each commit, forcing write-through of any disk write cache)
</P
></LI
><LI
><P
> <TT
CLASS="LITERAL"
>open_sync</TT
> (write WAL files with <CODE
CLASS="FUNCTION"
>open()</CODE
> option <TT
CLASS="SYMBOL"
>O_SYNC</TT
>)
</P
></LI
></UL
><P
> The <TT
CLASS="LITERAL"
>open_</TT
>* options also use <TT
CLASS="LITERAL"
>O_DIRECT</TT
> 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 <TT
CLASS="LITERAL"
>fdatasync</TT
> 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 <A
HREF="wal-reliability.html"
>Section 28.1</A
>.
This parameter can only be set in the <TT
CLASS="FILENAME"
>postgresql.conf</TT
>
file or on the server command line.
</P
></DD
><DT
><A
NAME="GUC-FULL-PAGE-WRITES"
></A
><TT
CLASS="VARNAME"
>full_page_writes</TT
> (<TT
CLASS="TYPE"
>boolean</TT
>)</DT
><DD
><P
> When this parameter is on, the <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 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 a price
in 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.)
</P
><P
> Turning this parameter off speeds normal operation, but
might lead to a corrupt database after an operating system crash
or power failure. The risks are similar to turning off
<TT
CLASS="VARNAME"
>fsync</TT
>, though smaller. It might be safe to turn off
this parameter if you have hardware (such as a battery-backed disk
controller) or file-system software that reduces
the risk of partial page writes to an acceptably low level (e.g., ReiserFS 4).
</P
><P
> Turning off this parameter does not affect use of
WAL archiving for point-in-time recovery (PITR)
(see <A
HREF="continuous-archiving.html"
>Section 24.3</A
>).
</P
><P
> This parameter can only be set in the <TT
CLASS="FILENAME"
>postgresql.conf</TT
>
file or on the server command line.
The default is <TT
CLASS="LITERAL"
>on</TT
>.
</P
></DD
><DT
><A
NAME="GUC-WAL-BUFFERS"
></A
><TT
CLASS="VARNAME"
>wal_buffers</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
> The amount of memory used in shared memory for WAL data. The
default is 64 kilobytes (<TT
CLASS="LITERAL"
>64kB</TT
>). The setting need only
be large enough to hold the amount of WAL data generated by one
typical transaction, since the data is written out to disk at
every transaction commit. This parameter can only be set at server
start.
</P
><P
> Increasing this parameter might cause <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
to request more <SPAN
CLASS="SYSTEMITEM"
>System V</SPAN
> shared
memory than your operating system's default configuration
allows. See <A
HREF="kernel-resources.html#SYSVIPC"
>Section 17.4.1</A
> for information on how to
adjust those parameters, if necessary.
</P
></DD
><DT
><A
NAME="GUC-WAL-WRITER-DELAY"
></A
><TT
CLASS="VARNAME"
>wal_writer_delay</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
> Specifies the delay between activity rounds for the WAL writer.
In each round the writer will flush WAL to disk. It then sleeps for
<TT
CLASS="VARNAME"
>wal_writer_delay</TT
> milliseconds, and repeats. The default
value is 200 milliseconds (<TT
CLASS="LITERAL"
>200ms</TT
>). Note that on many
systems, the effective resolution of sleep delays is 10 milliseconds;
setting <TT
CLASS="VARNAME"
>wal_writer_delay</TT
> 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
<TT
CLASS="FILENAME"
>postgresql.conf</TT
> file or on the server command line.
</P
></DD
><DT
><A
NAME="GUC-COMMIT-DELAY"
></A
><TT
CLASS="VARNAME"
>commit_delay</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
> Time delay between writing a commit record to the WAL buffer
and flushing the buffer out to disk, in microseconds. A
nonzero delay can allow multiple transactions to be committed
with only one <CODE
CLASS="FUNCTION"
>fsync()</CODE
> system call, 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
<TT
CLASS="VARNAME"
>commit_siblings</TT
> other transactions are
active at the instant that a server process has written its
commit record. The default is zero (no delay).
</P
></DD
><DT
><A
NAME="GUC-COMMIT-SIBLINGS"
></A
><TT
CLASS="VARNAME"
>commit_siblings</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
> Minimum number of concurrent open transactions to require
before performing the <TT
CLASS="VARNAME"
>commit_delay</TT
> 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.
</P
></DD
></DL
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="RUNTIME-CONFIG-WAL-CHECKPOINTS"
>18.5.2. Checkpoints</A
></H2
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><A
NAME="GUC-CHECKPOINT-SEGMENTS"
></A
><TT
CLASS="VARNAME"
>checkpoint_segments</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
> 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 <TT
CLASS="FILENAME"
>postgresql.conf</TT
>
file or on the server command line.
</P
></DD
><DT
><A
NAME="GUC-CHECKPOINT-TIMEOUT"
></A
><TT
CLASS="VARNAME"
>checkpoint_timeout</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
> Maximum time between automatic WAL checkpoints, in
seconds. The default is five minutes (<TT
CLASS="LITERAL"
>5min</TT
>).
Increasing this parameter can increase the amount of time needed
for crash recovery.
This parameter can only be set in the <TT
CLASS="FILENAME"
>postgresql.conf</TT
>
file or on the server command line.
</P
></DD
><DT
><A
NAME="GUC-CHECKPOINT-COMPLETION-TARGET"
></A
><TT
CLASS="VARNAME"
>checkpoint_completion_target</TT
> (<TT
CLASS="TYPE"
>floating point</TT
>)</DT
><DD
><P
> Specifies the target length of checkpoints, as a fraction of
the checkpoint interval. The default is 0.5.
This parameter can only be set in the <TT
CLASS="FILENAME"
>postgresql.conf</TT
>
file or on the server command line.
</P
></DD
><DT
><A
NAME="GUC-CHECKPOINT-WARNING"
></A
><TT
CLASS="VARNAME"
>checkpoint_warning</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
> 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
<TT
CLASS="VARNAME"
>checkpoint_segments</TT
> ought to be raised). The default is
30 seconds (<TT
CLASS="LITERAL"
>30s</TT
>). Zero disables the warning.
This parameter can only be set in the <TT
CLASS="FILENAME"
>postgresql.conf</TT
>
file or on the server command line.
</P
></DD
></DL
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="RUNTIME-CONFIG-WAL-ARCHIVING"
>18.5.3. Archiving</A
></H2
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><A
NAME="GUC-ARCHIVE-MODE"
></A
><TT
CLASS="VARNAME"
>archive_mode</TT
> (<TT
CLASS="TYPE"
>boolean</TT
>)</DT
><DD
><P
> When <TT
CLASS="VARNAME"
>archive_mode</TT
> is enabled, completed WAL segments
can be sent to archive storage by setting
<A
HREF="runtime-config-wal.html#GUC-ARCHIVE-COMMAND"
>archive_command</A
>.
<TT
CLASS="VARNAME"
>archive_mode</TT
> and <TT
CLASS="VARNAME"
>archive_command</TT
> are
separate variables so that <TT
CLASS="VARNAME"
>archive_command</TT
> can be
changed without leaving archiving mode.
This parameter can only be set at server start.
</P
></DD
><DT
><A
NAME="GUC-ARCHIVE-COMMAND"
></A
><TT
CLASS="VARNAME"
>archive_command</TT
> (<TT
CLASS="TYPE"
>string</TT
>)</DT
><DD
><P
> The shell command to execute to archive a completed segment of
the WAL file series. Any <TT
CLASS="LITERAL"
>%p</TT
> in the string is
replaced by the path name of the file to archive, and any
<TT
CLASS="LITERAL"
>%f</TT
> is replaced by the file name only.
(The path name is relative to the working directory of the server,
i.e., the cluster's data directory.)
Use <TT
CLASS="LITERAL"
>%%</TT
> to embed an actual <TT
CLASS="LITERAL"
>%</TT
> character in the
command. For more information see <A
HREF="continuous-archiving.html#BACKUP-ARCHIVING-WAL"
>Section 24.3.1</A
>.
This parameter can only be set in the <TT
CLASS="FILENAME"
>postgresql.conf</TT
>
file or on the server command line. It is ignored unless
<TT
CLASS="VARNAME"
>archive_mode</TT
> was enabled at server start.
If <TT
CLASS="VARNAME"
>archive_command</TT
> is an empty string (the default) while
<TT
CLASS="VARNAME"
>archive_mode</TT
> is enabled, then WAL archiving is temporarily
disabled, but the server continues to accumulate WAL segment files in
the expectation that a command will soon be provided.
</P
><P
> It is important for the command to return a zero exit status if
and only if it succeeds. Examples:
</P><PRE
CLASS="PROGRAMLISTING"
>archive_command = 'cp "%p" /mnt/server/archivedir/"%f"'
archive_command = 'copy "%p" "C:\\server\\archivedir\\%f"' # Windows</PRE
><P>
</P
></DD
><DT
><A
NAME="GUC-ARCHIVE-TIMEOUT"
></A
><TT
CLASS="VARNAME"
>archive_timeout</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
> The <A
HREF="runtime-config-wal.html#GUC-ARCHIVE-COMMAND"
>archive_command</A
> is only invoked on
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 put a limit on how old unarchived
data can be, you can set <TT
CLASS="VARNAME"
>archive_timeout</TT
> 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. 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
<TT
CLASS="VARNAME"
>archive_timeout</TT
> — it will bloat your archive
storage. <TT
CLASS="VARNAME"
>archive_timeout</TT
> settings of a minute or so are
usually reasonable. This parameter can only be set in the
<TT
CLASS="FILENAME"
>postgresql.conf</TT
> file or on the server command line.
</P
></DD
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