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Fix some misstatements in WAL parameter discussion.

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This commit is contained in:
Tom Lane 2002-11-02 22:23:01 +00:00
parent 2a01b05936
commit 77fcc1cbae
1 changed files with 30 additions and 43 deletions
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doc/src/sgml/wal.sgml
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@ -1,4 +1,4 @@
<!-- $Header: /cvsroot/pgsql/doc/src/sgml/wal.sgml,v 1.20 2002/09/06 20:26:00 momjian Exp $ -->
<!-- $Header: /cvsroot/pgsql/doc/src/sgml/wal.sgml,v 1.21 2002/11/02 22:23:01 tgl Exp $ -->
<chapter id="wal">
<title>Write-Ahead Logging (<acronym>WAL</acronym>)</title>
@ -222,33 +222,6 @@
configuration parameters.
</para>
<para>
There are two commonly used <acronym>WAL</acronym> functions:
<function>LogInsert</function> and <function>LogFlush</function>.
<function>LogInsert</function> is used to place a new record into
the <acronym>WAL</acronym> buffers in shared memory. If there is no
space for the new record, <function>LogInsert</function> will have
to write (move to kernel cache) a few filled <acronym>WAL</acronym>
buffers. This is undesirable because <function>LogInsert</function>
is used on every database low level modification (for example,
tuple 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>WAL</acronym> buffers may
also force the creation of a new log segment, which takes even more
time. Normally, <acronym>WAL</acronym> buffers should be written
and flushed by a <function>LogFlush</function> 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, <function>LogFlush</function> requests may
not occur often enough to prevent <acronym>WAL</acronym> buffers
being written by <function>LogInsert</function>. On such systems
one should increase the number of <acronym>WAL</acronym> buffers by
modifying the <filename>postgresql.conf</filename> <varname>
WAL_BUFFERS</varname> parameter. The default number of <acronym>
WAL</acronym> buffers is 8. Increasing this value will
correspondingly increase shared memory usage.
</para>
<para>
<firstterm>Checkpoints</firstterm> are points in the sequence of
transactions at which it is guaranteed that the data files have
@ -265,19 +238,6 @@
or removed.)
</para>
<para>
The checkpoint maker is also able to create a few log segments for
future use, so as to avoid the need for
<function>LogInsert</function> or <function>LogFlush</function> to
spend time in creating them. (If that happens, the entire database
system will be delayed by the creation operation, so it's better if
the files can be created in the checkpoint maker, which is not on
anyone's critical path.)
By default a new 16MB segment file is created only if more than 75% of
the current segment has been used. This is inadequate if the system
generates more than 4MB of log output between checkpoints.
</para>
<para>
The postmaster spawns a special backend process every so often
to create the next checkpoint. A checkpoint is created every
@ -303,16 +263,43 @@
<para>
There will be at least one 16MB segment file, and will normally
not be more than <varname>CHECKPOINT_SEGMENTS</varname>
not be more than 2 * <varname>CHECKPOINT_SEGMENTS</varname>
+ 1 files. You can use this to estimate space requirements for
WAL. Ordinarily, when old log segment files are no longer needed,
they are recycled (renamed to become the next sequential future
segments). If, due to a short-term peak of log output rate, there
are more than <varname>CHECKPOINT_SEGMENTS</varname> + 1 segment files,
are more than 2 * <varname>CHECKPOINT_SEGMENTS</varname> + 1 segment files,
the unneeded segment files will be deleted instead of recycled until the
system gets back under this limit.
</para>
<para>
There are two commonly used <acronym>WAL</acronym> functions:
<function>LogInsert</function> and <function>LogFlush</function>.
<function>LogInsert</function> is used to place a new record into
the <acronym>WAL</acronym> buffers in shared memory. If there is no
space for the new record, <function>LogInsert</function> will have
to write (move to kernel cache) a few filled <acronym>WAL</acronym>
buffers. This is undesirable because <function>LogInsert</function>
is used on every database low level modification (for example,
tuple 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>WAL</acronym> buffers may
also force the creation of a new log segment, which takes even more
time. Normally, <acronym>WAL</acronym> buffers should be written
and flushed by a <function>LogFlush</function> 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, <function>LogFlush</function> requests may
not occur often enough to prevent <acronym>WAL</acronym> buffers
being written by <function>LogInsert</function>. On such systems
one should increase the number of <acronym>WAL</acronym> buffers by
modifying the <filename>postgresql.conf</filename> <varname>
WAL_BUFFERS</varname> parameter. The default number of <acronym>
WAL</acronym> buffers is 8. Increasing this value will
correspondingly increase shared memory usage.
</para>
<para>
The <varname>COMMIT_DELAY</varname> parameter defines for how many
microseconds the backend will sleep after writing a commit