pg_standby — supports the creation of a PostgreSQL warm standby server
pg_standby
[option...] archivelocation nextwalfile walfilepath [restartwalfile]
pg_standby supports creation of a “warm standby” database server. It is designed to be a production-ready program, as well as a customizable template should you require specific modifications.
pg_standby is designed to be a
waiting restore_command, which is
needed to turn a standard archive recovery into a warm standby
operation. Other configuration is required as well, all of which is
described in the main server manual (see Section 26.2).
To configure a standby server to use pg_standby, put this into its recovery.conf configuration file:
restore_command = 'pg_standby archiveDir %f %p %r'
where archiveDir is
the directory from which WAL segment files should be restored.
If restartwalfile is
specified, normally by using the %r
macro, then all WAL files logically preceding this file will be
removed from archivelocation. This minimizes the
number of files that need to be retained, while preserving
crash-restart capability. Use of this parameter is appropriate if
the archivelocation is a
transient staging area for this particular standby server, but
not when the archivelocation is intended as a
long-term WAL archive area.
pg_standby assumes that
archivelocation is a
directory readable by the server-owning user. If restartwalfile (or -k) is specified, the archivelocation directory must be
writable too.
There are two ways to fail over to a “warm standby” database server when the master server fails:
In smart failover, the server is brought up after applying all
WAL files available in the archive. This results in zero data loss,
even if the standby server has fallen behind, but if there is a lot
of unapplied WAL it can be a long time before the standby server
becomes ready. To trigger a smart failover, create a trigger file
containing the word smart, or just
create it and leave it empty.
In fast failover, the server is brought up immediately. Any WAL
files in the archive that have not yet been applied will be
ignored, and all transactions in those files are lost. To trigger a
fast failover, create a trigger file and write the word
fast into it. pg_standby can also be configured to execute a
fast failover automatically if no new WAL file appears within a
defined interval.
pg_standby accepts the following command-line arguments:
-cUse cp or copy command to restore WAL files from archive.
This is the only supported behavior so this option is useless.
-dPrint lots of debug logging output on stderr.
-kRemove files from archivelocation so that no more
than this many WAL files before the current one are kept in the
archive. Zero (the default) means not to remove any files from
archivelocation. This
parameter will be silently ignored if restartwalfile is specified, since
that specification method is more accurate in determining the
correct archive cut-off point. Use of this parameter is
deprecated as of
PostgreSQL 8.3; it is safer and
more efficient to specify a restartwalfile parameter. A too
small setting could result in removal of files that are still
needed for a restart of the standby server, while a too large
setting wastes archive space.
-r maxretriesSet the maximum number of times to retry the copy command if it
fails (default 3). 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 standby
server. This will be interpreted as end of recovery and the standby
will come up fully as a result.
-s sleeptimeSet the number of seconds (up to 60, default 5) to sleep between tests to see if the WAL file to be restored is available in the archive yet. The default setting is not necessarily recommended; consult Section 26.2 for discussion.
-t triggerfileSpecify a trigger file whose presence should cause failover. It
is recommended that you use a structured file name to avoid
confusion as to which server is being triggered when multiple
servers exist on the same system; for example /tmp/pgsql.trigger.5432.
-V--versionPrint the pg_standby version and exit.
-w maxwaittimeSet the maximum number of seconds to wait for the next WAL file, after which a fast failover will be performed. A setting of zero (the default) means wait forever. The default setting is not necessarily recommended; consult Section 26.2 for discussion.
-?--helpShow help about pg_standby command line arguments, and exit.
pg_standby is designed to work with PostgreSQL 8.2 and later.
PostgreSQL 8.3 provides the
%r macro, which is designed to let
pg_standby know the last file it
needs to keep. With PostgreSQL
8.2, the -k option must be used if
archive cleanup is required. This option remains available in 8.3,
but its use is deprecated.
PostgreSQL 8.4 provides the
recovery_end_command option. Without
this option a leftover trigger file can be hazardous.
pg_standby is written in C and has an easy-to-modify source code, with specifically designated sections to modify for your own needs
On Linux or Unix systems, you might use:
archive_command = 'cp %p .../archive/%f' restore_command = 'pg_standby -d -s 2 -t /tmp/pgsql.trigger.5442 .../archive %f %p %r 2>>standby.log' recovery_end_command = 'rm -f /tmp/pgsql.trigger.5442'
where the archive directory is physically located on the standby
server, so that the archive_command is
accessing it across NFS, but the files are local to the standby
(enabling use of ln). This will:
produce debugging output in standby.log
sleep for 2 seconds between checks for next WAL file availability
stop waiting only when a trigger file called /tmp/pgsql.trigger.5442 appears, and perform
failover according to its content
remove the trigger file when recovery ends
remove no-longer-needed files from the archive directory
On Windows, you might use:
archive_command = 'copy %p ...\\archive\\%f' restore_command = 'pg_standby -d -s 5 -t C:\pgsql.trigger.5442 ...\archive %f %p %r 2>>standby.log' recovery_end_command = 'del C:\pgsql.trigger.5442'
Note that backslashes need to be doubled in the archive_command, but not in the restore_command or recovery_end_command. This will:
use the copy command to restore WAL
files from archive
produce debugging output in standby.log
sleep for 5 seconds between checks for next WAL file availability
stop waiting only when a trigger file called C:\pgsql.trigger.5442 appears, and perform
failover according to its content
remove the trigger file when recovery ends
remove no-longer-needed files from the archive directory
The copy command on Windows sets
the final file size before the file is completely copied, which
would ordinarily confuse pg_standby. Therefore pg_standby waits sleeptime seconds once it sees the
proper file size. GNUWin32's cp sets
the file size only after the file copy is complete.
Since the Windows example uses copy
at both ends, either or both servers might be accessing the archive
directory across the network.
Simon Riggs <simon@2ndquadrant.com>
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