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WAL documentation, from Oliver Elphick and Vadim Mikheev.

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Peter Eisentraut 2001-01-24 23:15:19 +00:00
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<book id="admin">
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&manage-ag;
&user-manag;
&backup;
&wal;
&recovery;
&regress;
&release;

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<!-- $Header: /cvsroot/pgsql/doc/src/sgml/filelist.sgml,v 1.6 2001/01/24 23:15:19 petere Exp $ -->
<!entity about SYSTEM "about.sgml">
<!entity history SYSTEM "history.sgml">
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<!entity release SYSTEM "release.sgml">
<!entity runtime SYSTEM "runtime.sgml">
<!entity user-manag SYSTEM "user-manag.sgml">
<!entity wal SYSTEM "wal.sgml">
<!-- programmer's guide -->
<!entity arch-pg SYSTEM "arch-pg.sgml">

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<Chapter Id="runtime">
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</para>
</sect2>
<sect2 id="runtime-config-wal">
<title>WAL</title>
<para>
See also <xref linkend="wal-configuration"> for details on WAL
tuning.
<variablelist>
<varlistentry>
<term>CHECKPOINT_TIMEOUT (<type>integer</type>)</term>
<listitem>
<para>
Frequency of automatic WAL checkpoints, in seconds.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>WAL_BUFFERS (<type>integer</type>)</term>
<listitem>
<para>
Number of buffers for WAL. This option can only be set at
server start.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>WAL_DEBUG (<type>integer</type>)</term>
<listitem>
<para>
If non-zero, turn on WAL-related debugging output on standard
error.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>WAL_FILES (<type>integer</type>)</term>
<listitem>
<para>
Number of log files that are created in advance at checkpoint
time. This option can only be set at server start.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</sect2>
<sect2 id="runtime-config-short">
<title>Short options</title>
<para>

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<chapter id="wal">
<title>Write-Ahead Logging (<acronym>WAL</acronym>)</title>
<note>
<title>Author</title>
<para>
Vadim Mikheev and Oliver Elphick
</para>
</note>
<sect1 id="wal-general">
<title>General Description</Title>
<para>
<firstterm>Write Ahead Logging</firstterm> (<acronym>WAL</acronym>)
is a standard approach to transaction logging. Its detailed
description may be found in most (if not all) books about
transaction processing. Briefly, <acronym>WAL</acronym>'s central
concept is that changes to data files (where tables and indices
reside) must be written only after those changes have been logged -
that is, when log records have been flushed to permanent
storage. When we follow this procedure, we do not need to flush
data pages to disk on every transaction commit, because we know
that in the event of a crash we will be able to recover the
database using the log: any changes that have not been applied to
the data pages will first be redone from the log records (this is
roll-forward recovery, also known as REDO) and then changes made by
uncommitted transactions will be removed from the data pages
(roll-backward recovery - UNDO).
</para>
<sect2 id="wal-benefits-now">
<title>Immediate Benefits of <acronym>WAL</acronym></title>
<para>
The first obvious benefit of using <acronym>WAL</acronym> is a
significantly reduced number of disk writes, since only the log
file needs to be flushed to disk at the time of transaction
commit; in multi-user environments, commits of many transactions
may be accomplished with a single <function>fsync()</function> of
the log file. Furthermore, the log file is written sequentially,
and so the cost of syncing the log is much less than the cost of
flushing the data pages.
</para>
<para>
The next benefit is consistency of the data pages. The truth is
that, before <acronym>WAL</acronym>,
<productname>PostgreSQL</productname> was never able to guarantee
consistency in the case of a crash. Before
<acronym>WAL</acronym>, any crash during writing could result in:
<orderedlist>
<listitem>
<simpara>index tuples pointing to non-existent table rows</simpara>
</listitem>
<listitem>
<simpara>index tuples lost in split operations</simpara>
</listitem>
<listitem>
<simpara>totally corrupted table or index page content, because
of partially written data pages</simpara>
</listitem>
</orderedlist>
Problems with indices (problems 1 and 2) could possibly have been
fixed by additional <function>fsync()</function> calls, but it is
not obvious how to handle the last case without
<acronym>WAL</acronym>; <acronym>WAL</acronym> saves the entire
data page content in the log if that is required to ensure page
consistency for after-crash recovery.
</para>
</sect2>
<sect2 id="wal-benefits-later">
<title>Future Benefits</title>
<para>
In this first release of <acronym>WAL</acronym>, UNDO operation is
not implemented, because of lack of time. This means that changes
made by aborted transactions will still occupy disk space and that
we still need a permanent <filename>pg_log</filename> file to hold
the status of transactions, since we are not able to re-use
transaction identifiers. Once UNDO is implemented,
<filename>pg_log</filename> will no longer be required to be
permanent; it will be possible to remove
<filename>pg_log</filename> at shutdown, split it into segments
and remove old segments.
</para>
<para>
With UNDO, it will also be possible to implement
<firstterm>savepoints</firstterm> to allow partial rollback of
invalid transaction operations (parser errors caused by mistyping
commands, insertion of duplicate primary/unique keys and so on)
with the ability to continue or commit valid operations made by
the transaction before the error. At present, any error will
invalidate the whole transaction and require a transaction abort.
</para>
<para>
<acronym>WAL</acronym> offers the opportunity for a new method for
database on-line backup and restore (<acronym>BAR</acronym>). To
use this method, one would have to make periodic saves of data
files to another disk, a tape or another host and also archive the
<acronym>WAL</acronym> log files. The database file copy and the
archived log files could be used to restore just as if one were
restoring after a crash. Each time a new database file copy was
made the old log files could be removed. Implementing this
facility will require the logging of data file and index creation
and deletion; it will also require development of a method for
copying the data files (operating system copy commands are not
suitable).
</para>
</sect2>
</sect1>
<sect1 id="wal-implementation">
<title>Implementation</title>
<para>
<acronym>WAL</acronym> is automatically enabled from release 7.1
onwards. No action is required from the administrator with the
exception of ensuring that the additional disk-space requirements
of the <acronym>WAL</acronym> logs are met, and that any necessary
tuning is done (see <xref linkend="wal-configuration">).
</para>
<para>
<acronym>WAL</acronym> logs are stored in the directory
<Filename><replaceable>$PGDATA</replaceable>/pg_xlog</Filename>, as
a set of segment files, each 16 MB in size. Each segment is
divided into 8 kB pages. The log record headers are described in
<filename>access/xlog.h</filename>; record content is dependent on
the type of event that is being logged. Segment files are given
sequential numbers as names, starting at
<filename>0000000000000000</filename>. The numbers do not wrap, at
present, but it should take a very long time to exhaust the
available stock of numbers.
</para>
<para>
The <acronym>WAL</acronym> buffers and control structure are in
shared memory, and are handled by the backends; they are protected
by spinlocks. The demand on shared memory is dependent on the
number of buffers; the default size of the <acronym>WAL</acronym>
buffers is 64 kB.
</para>
<para>
It is of advantage if the log is located on another disk than the
main database files. This may be achieved by moving the directory,
<filename>pg_xlog</filename>, to another location (while the
postmaster is shut down, of course) and creating a symbolic link
from the original location in <replaceable>$PGDATA</replaceable> to
the new location.
</para>
<para>
The aim of <acronym>WAL</acronym>, to ensure that the log is
written before database records are altered, may be subverted by
disk drives that falsely report a successful write to the kernel,
when, in fact, they have only cached the data and not yet stored it
on the disk. A power failure in such a situation may still lead to
irrecoverable data corruption; administrators should try to ensure
that disks holding <productname>PostgreSQL</productname>'s data and
log files do not make such false reports.
</para>
<sect2 id="wal-recovery">
<title>Database Recovery with <acronym>WAL</acronym></title>
<para>
After a checkpoint has been made and the log flushed, the
checkpoint's position is saved in the file
<filename>pg_control</filename>. Therefore, when recovery is to be
done, the backend first reads <filename>pg_control</filename> and
then the checkpoint record; next it reads the redo record, whose
position is saved in the checkpoint, and begins the REDO operation.
Because the entire content of the pages is saved in the log on the
first page modification after a checkpoint, the pages will be first
restored to a consistent state.
</para>
<para>
Using <filename>pg_control</filename> to get the checkpoint
position speeds up the recovery process, but to handle possible
corruption of <filename>pg_control</filename>, we should actually
implement the reading of existing log segments in reverse order --
newest to oldest -- in order to find the last checkpoint. This has
not yet been done in release 7.1.
</para>
</sect2>
</sect1>
<sect1 id="wal-configuration">
<title><acronym>WAL</acronym> Configuration</title>
<para>
There are several <acronym>WAL</acronym>-related parameters that
affect database performance. This section explains their use.
Consult <xref linkend="runtime-config"> for details about setting
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 and the operation is supposed to be as fast as
possible; what is worse, writing <acronym>WAL</acronym> buffers may
also cause 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 <varname>WAL_BUFFERS</varname> parameter. The default
number of <acronym>WAL</acronym> buffers is 8. Increasing this
value will have an impact on 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
been updated with all information logged 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. As result, in
the event of a crash, the recoverer knows from what record in the
log (known as the redo record) it should start the REDO operation,
since any changes made to data files before that record are already
on disk. After a checkpoint has been made, any log segments written
before the redo record are removed, so checkpoints are used to free
disk space in the <acronym>WAL</acronym> directory. (When
<acronym>WAL</acronym>-based <acronym>BAR</acronym> is implemented,
the log segments can be archived instead of just being removed.)
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.
</para>
<para>
The <acronym>WAL</acronym> log is held on the disk as a set of 16
MB files called <firstterm>segments</firstterm>. By default a new
segment is created only if more than 75% of the current segment is
used. One can instruct the server to create up to 64 log segments
at checkpoint time by modifying the <varname>WAL_FILES</varname>
configuration parameter.
</para>
<para>
For faster after-crash recovery, it would be better to create
checkpoints more often. However, one should balance this against
the cost of flushing dirty data pages; in addition, to ensure data
page consistency, the first modification of a data page after each
checkpoint results in logging the entire page content, thus
increasing output to log and the log's size.
</para>
<para>
By default, the postmaster spawns a special backend process to
create the next checkpoint 300 seconds after the previous
checkpoint's creation. One can change this interval by modifying
the <varname>CHECKPOINT_TIMEOUT</varname> parameter. It is also
possible to force a checkpoint by using the SQL command
<command>CHECKPOINT</command>.
</para>
<para>
Setting the <varname>WAL_DEBUG</varname> parameter to any non-zero
value will result in each <function>LogInsert</function> and
<function>LogFlush</function> <acronym>WAL</acronym> call being
logged to standard error. At present, it makes no difference what
the non-zero value is. This option may be replaced by a more
general mechanism in the future.
</para>
<para>
The <varname>COMMIT_DELAY</varname> parameter defines for how long
the backend will be forced to sleep after writing a commit record
to the log with <function>LogInsert</function> call but before
performing a <function>LogFlush</function>. This delay allows other
backends to add their commit records to the log so as to have all
of them flushed with a single log sync. Unfortunately, this
mechanism is not fully implemented at release 7.1, so there is at
present no point in changing this parameter from its default value
of 5 microseconds.
</para>
</sect1>
</chapter>
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