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<title>26.3.WAL Configuration</title>
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="wal-configuration"></a>26.3.<acronym class="acronym">WAL</acronym> Configuration</h2></div></div></div>
<p> There are several <acronym class="acronym">WAL</acronym>-related configuration parameters that
affect database performance. This section explains their use.
Consult <a href="runtime-config.html" title="Chapter17.Server Configuration">Chapter17, <i>Server Configuration</i></a> for general information about
setting server configuration parameters.
</p>
<p> <em class="firstterm">Checkpoints</em><a name="id673773"></a>
are points in the sequence of transactions at which it is guaranteed
that the data files have been updated with all information written before
the checkpoint. At checkpoint time, all dirty data pages are flushed to
disk and a special checkpoint record is written to the log file.
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 known to be already on disk. Hence, after
a checkpoint has been made, any log segments preceding the one containing
the redo record are no longer needed and can be recycled or removed. (When
<acronym class="acronym">WAL</acronym> archiving is being done, the log segments must be
archived before being recycled or removed.)
</p>
<p> The server's background writer process will automatically perform
a checkpoint every so often. A checkpoint is created every <a href="runtime-config-wal.html#guc-checkpoint-segments">checkpoint_segments</a> log segments, or every <a href="runtime-config-wal.html#guc-checkpoint-timeout">checkpoint_timeout</a> seconds, whichever comes first.
The default settings are 3 segments and 300 seconds respectively.
It is also possible to force a checkpoint by using the SQL command
<code class="command">CHECKPOINT</code>.
</p>
<p> Reducing <code class="varname">checkpoint_segments</code> and/or
<code class="varname">checkpoint_timeout</code> causes checkpoints to be done
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. If
<a href="runtime-config-wal.html#guc-full-page-writes">full_page_writes</a> 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.
</p>
<p> 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 that checkpoints
don't happen too often. As a simple sanity check on your checkpointing
parameters, you can set the <a href="runtime-config-wal.html#guc-checkpoint-warning">checkpoint_warning</a>
parameter. If checkpoints happen closer together than
<code class="varname">checkpoint_warning</code> seconds,
a message will be output to the server log recommending increasing
<code class="varname">checkpoint_segments</code>. 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 large <code class="command">COPY</code> transfers may cause a number of such warnings
to appear if you have not set <code class="varname">checkpoint_segments</code> high
enough.
</p>
<p> There will be at least one WAL segment file, and will normally
not be more than 2 * <code class="varname">checkpoint_segments</code> + 1
files. Each segment file is normally 16 MB (though this size can be
altered when building the server). You can use this to estimate space
requirements for <acronym class="acronym">WAL</acronym>.
Ordinarily, when old log segment files are no longer needed, they
are recycled (renamed to become the next segments in the numbered
sequence). If, due to a short-term peak of log output rate, there
are more than 2 * <code class="varname">checkpoint_segments</code> + 1
segment files, the unneeded segment files will be deleted instead
of recycled until the system gets back under this limit.
</p>
<p> There are two commonly used internal <acronym class="acronym">WAL</acronym> functions:
<code class="function">LogInsert</code> and <code class="function">LogFlush</code>.
<code class="function">LogInsert</code> is used to place a new record into
the <acronym class="acronym">WAL</acronym> buffers in shared memory. If there is no
space for the new record, <code class="function">LogInsert</code> will have
to write (move to kernel cache) a few filled <acronym class="acronym">WAL</acronym>
buffers. This is undesirable because <code class="function">LogInsert</code>
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, writing <acronym class="acronym">WAL</acronym> buffers may also force the
creation of a new log segment, which takes even more
time. Normally, <acronym class="acronym">WAL</acronym> buffers should be written
and flushed by a <code class="function">LogFlush</code> 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, <code class="function">LogFlush</code> requests may
not occur often enough to prevent <code class="function">LogInsert</code>
from having to do writes. On such systems
one should increase the number of <acronym class="acronym">WAL</acronym> buffers by
modifying the configuration parameter <a href="runtime-config-wal.html#guc-wal-buffers">wal_buffers</a>. The default number of <acronym class="acronym">WAL</acronym>
buffers is 8. Increasing this value will
correspondingly increase shared memory usage. When
<a href="runtime-config-wal.html#guc-full-page-writes">full_page_writes</a> is set and the system is very busy,
setting this value higher will help smooth response times during the
period immediately following each checkpoint.
</p>
<p> The <a href="runtime-config-wal.html#guc-commit-delay">commit_delay</a> parameter defines for how many
microseconds the server process will sleep after writing a commit
record to the log with <code class="function">LogInsert</code> but before
performing a <code class="function">LogFlush</code>. This delay allows other
server processes to add their commit records to the log so as to have all
of them flushed with a single log sync. No sleep will occur if
<a href="runtime-config-wal.html#guc-fsync">fsync</a>
is not enabled, nor if fewer than <a href="runtime-config-wal.html#guc-commit-siblings">commit_siblings</a>
other sessions are currently in active transactions; this avoids
sleeping when it's unlikely that any other session will commit soon.
Note that on most platforms, the resolution of a sleep request is
ten milliseconds, so that any nonzero <code class="varname">commit_delay</code>
setting between 1 and 10000 microseconds would have the same effect.
Good values for these parameters are not yet clear; experimentation
is encouraged.
</p>
<p> The <a href="runtime-config-wal.html#guc-wal-sync-method">wal_sync_method</a> parameter determines how
<span class="productname">PostgreSQL</span> will ask the kernel to force
<acronym class="acronym">WAL</acronym> updates out to disk.
All the options should be the same as far as reliability goes,
but it's quite platform-specific which one will be the fastest.
Note that this parameter is irrelevant if <code class="varname">fsync</code>
has been turned off.
</p>
<p> Enabling the <a href="runtime-config-developer.html#guc-wal-debug">wal_debug</a> configuration parameter
(provided that <span class="productname">PostgreSQL</span> has been
compiled with support for it) will result in each
<code class="function">LogInsert</code> and <code class="function">LogFlush</code>
<acronym class="acronym">WAL</acronym> call being logged to the server log. This
option may be replaced by a more general mechanism in the future.
</p>
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