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<sect1 id="pgstandby">
<title>pg_standby</title>
<indexterm zone="pgstandby">
<primary>pg_standby</primary>
</indexterm>
<para>
<literal>pg_standby</literal> allows the creation of a Warm Standby server.
Other configuration is required as well, all of which is described in the
main server manual.
</para>
<para>
The program is designed to be a wait-for <literal>restore_command</literal>,
required to turn a normal archive recovery into a Warm Standby. Within the
<literal>restore_command</literal> of the <literal>recovery.conf</literal>
you could configure <literal>pg_standby</literal> in the following way:
</para>
<programlisting>
restore_command = 'pg_standby archiveDir %f %p %r'
</programlisting>
<para>
which would be sufficient to define that files will be restored from
archiveDir.
</para>
<para>
<literal>pg_standby</literal> features include:
</para>
<itemizedlist>
<listitem>
<para>
It is written in C. So it is very portable
and easy to install.
</para>
</listitem>
<listitem>
<para>
Supports copy or link from a directory (only)
</para>
</listitem>
<listitem>
<para>
Source easy to modify, with specifically designated
sections to modify for your own needs, allowing
interfaces to be written for additional Backup Archive Restore
(BAR) systems
</para>
</listitem>
<listitem>
<para>
Already tested on Linux and Windows
</para>
</listitem>
</itemizedlist>
<sect2>
<title>Usage</title>
<para>
<literal>pg_standby</literal> should be used within the
<literal>restore_command</literal> of the <literal>recovery.conf</literal>
file.
</para>
<para>
The basic usage should be like this:
</para>
<programlisting>
restore_command = 'pg_standby archiveDir %f %p'
</programlisting>
<para>
with the pg_standby command usage as
</para>
<programlisting>
pg_standby [OPTION]... ARCHIVELOCATION NEXTWALFILE XLOGFILEPATH [RESTARTWALFILE]
</programlisting>
<para>
When used within the <literal>restore_command</literal> the %f and %p macros
will provide the actual file and path required for the restore/recovery.
</para>
<para>
<literal>pg_standby</literal> assumes that <literal>ARCHIVELOCATION</literal>
is a directory accessible by the server-owning user.
</para>
<para>
If <literal>RESTARTWALFILE</literal> is specified, typically by using the
<literal>%r</literal> option, then all files prior to this file will be
removed from <literal>ARCHIVELOCATION</literal>. This then minimises the
number of files that need to be held, whilst at the same time maintaining
restart capability. This capability additionally assumes that
<literal>ARCHIVELOCATION</literal> directory is writable.
</para>
<table>
<title>Options</title>
<tgroup cols="2">
<tbody>
<row>
<entry>-c</entry>
<entry> use copy/cp command to restore WAL files from archive</entry>
</row>
<row>
<entry>-d</entry>
<entry>debug/logging option.</entry>
</row>
<row>
<entry>-k numfiles</entry>
<entry>
<para>
Cleanup files in the archive so that we maintain no more than this
many files in the archive. This parameter will be silently ignored if
<literal>RESTARTWALFILE</literal> is specified, since that
specification method is more accurate in determining the correct
cut-off point in archive.
</para>
<para>
You should be wary against setting this number too low,
since this may mean you cannot restart the standby. This
is because the last restartpoint marked in the WAL files
may be many files in the past and can vary considerably.
This should be set to a value exceeding the number of WAL
files that can be recovered in 2*checkpoint_timeout seconds,
according to the value in the warm standby postgresql.conf.
It is wholly unrelated to the setting of checkpoint_segments
on either primary or standby.
</para>
<para>
Setting <literal>numfiles</literal> to be zero will disable deletion
of files from <literal>ARCHIVELOCATION</literal>.
</para>
<para>
If in doubt, use a large value or do not set a value at all.
</para>
<para>
If you specify neither <literal>RESTARTWALFILE</> nor <literal>-k</>,
then <literal>-k 0</> will be assumed, i.e. keep all files in archive.
</para>
</entry>
</row>
<row>
<entry>-l</entry>
<entry>
<para>
use ln command to restore WAL files from archive
WAL files will remain in archive
</para>
<para>
Link is more efficient, but the default is copy to allow you to
maintain the WAL archive for recovery purposes as well as
high-availability. The default setting is not necessarily recommended,
consult the main database server manual for discussion.
</para>
<para>
This option uses the Windows Vista command mklink
to provide a file-to-file symbolic link. -l will
not work on versions of Windows prior to Vista.
Use the -c option instead.
see <ulink url="http://en.wikipedia.org/wiki/NTFS_symbolic_link"></ulink>
</para>
</entry>
</row>
<row>
<entry>-r maxretries</entry>
<entry>
<para>
the maximum number of times to retry the restore command if it
fails. After each failure, we wait for sleeptime * num_retries
so that the wait time increases progressively, so by default
we will wait 5 secs, 10 secs then 15 secs before reporting
the failure back to the database server. This will be
interpreted as and end of recovery and the Standby will come
up fully as a result. <literal>Default=3 Min=0</literal>
</para>
</entry>
</row>
<row>
<entry>-s sleeptime</entry>
<entry>
the number of seconds to sleep between testing to see
if the file to be restored is available in the archive yet.
The default setting is not necessarily recommended,
consult the main database server manual for discussion.
<literal>Default=5, Min=1, Max=60</literal>
</entry>
</row>
<row>
<entry>-t triggerfile</entry>
<entry>
the presence of the triggerfile will cause recovery to end
whether or not the next file is available
It is recommended that you use a structured filename to
avoid confusion as to which server is being triggered
when multiple servers exist on same system.
e.g. /tmp/pgsql.trigger.5432
</entry>
</row>
<row>
<entry>-w maxwaittime</entry>
<entry>
the maximum number of seconds to wait for the next file,
after which recovery will end and the Standby will come up.
The default setting is not necessarily recommended,
consult the main database server manual for discussion. A setting of
zero means wait forever.
<literal>Default=0, Min=0</literal>
</entry>
</row>
</tbody>
</tgroup>
</table>
<note>
<para>
<literal>--help</literal> is not supported since
<literal>pg_standby</literal> is not intended for interactive use, except
during development and testing.
</para>
</note>
</sect2>
<sect2>
<title>Supported versions</title>
<para>
<literal>pg_standby</literal> is designed to work with PostgreSQL 8.2 and
and later. It is currently compatible across minor changes between the way
8.3 and 8.2 operate.
</para>
<para>
PostgreSQL 8.3 provides the <literal>%r</literal> command line substitution,
designed to let <literal>pg_standby</literal> know the last file it needs to
keep. If the last parameter is omitted, no error is generated, allowing
<literal>pg_standby</literal> to function correctly with PostgreSQL 8.2
also. With PostgreSQL 8.2, the <literal>-k</literal> option must be used if
archive cleanup is required. This option remains available in 8.3.
</para>
</sect2>
<sect2>
<title>Additional design notes</title>
<para>
The use of a move command seems like it would be a good idea, but this would
prevent recovery from being restartable. Also, the last WAL file is always
requested twice from the archive.
</para>
</sect2>
<sect2>
<title>Examples</title>
<itemizedlist>
<listitem>
<para>Example on Linux</para>
<programlisting>
archive_command = 'cp %p ../archive/%f'
restore_command = 'pg_standby -l -d -k 255 -r 2 -s 2 -w 0 -t /tmp/pgsql.trigger.5442 $PWD/../archive %f %p 2>> standby.log'
</programlisting>
<para>
which will
</para>
<itemizedlist>
<listitem><para>use a ln command to restore WAL files from archive</para></listitem>
<listitem><para>produce logfile output in standby.log</para></listitem>
<listitem><para>keep the last 255 full WAL files, plus the current one</para></listitem>
<listitem><para>sleep for 2 seconds between checks for next WAL file is full</para></listitem>
<listitem><para>never timeout if file not found</para></listitem>
<listitem><para>stop waiting when a trigger file called /tmp/pgsql.trigger.5442 appears</para></listitem>
</itemizedlist>
</listitem>
<listitem>
<para>
Example on Windows
</para>
<programlisting>
archive_command = 'copy %p ..\\archive\\%f'
</programlisting>
<para>
Note that backslashes need to be doubled in the archive_command, but
*not* in the restore_command, in 8.2, 8.1, 8.0 on Windows.
</para>
<programlisting>
restore_command = 'pg_standby -c -d -s 5 -w 0 -t C:\pgsql.trigger.5442 ..\archive %f %p 2>> standby.log'
</programlisting>
<para>
which will
</para>
<itemizedlist>
<listitem><para>use a copy command to restore WAL files from archive</para></listitem>
<listitem><para>produce logfile output in standby.log</para></listitem>
<listitem><para>sleep for 5 seconds between checks for next WAL file is full</para></listitem>
<listitem><para>never timeout if file not found</para></listitem>
<listitem><para>stop waiting when a trigger file called C:\pgsql.trigger.5442 appears</para></listitem>
</itemizedlist>
</listitem>
</itemizedlist>
</sect2>
</sect1>
|
