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Bruce Momjian
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pg_standby README 2006/12/08 Simon Riggs
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o What is pg_standby?
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pg_standby allows the creation of a Warm Standby server.
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It is designed to be a production-ready program, as well as a
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customisable template should you require specific modifications.
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Other configuration is required as well, all of which is
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described in the main server manual.
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The program is designed to be a wait-for restore_command,
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required to turn a normal archive recovery into a Warm Standby.
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Within the restore_command of the recovery.conf you could
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configure pg_standby in the following way:
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restore_command = 'pg_standby archiveDir %f %p %r'
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which would be sufficient to define that files will be restored
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from archiveDir.
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o features of pg_standby
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- pg_standby is written in C. So it is very portable
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and easy to install.
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- supports copy or link from a directory (only)
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- source easy to modify, with specifically designated
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sections to modify for your own needs, allowing
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interfaces to be written for additional Backup Archive Restore
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(BAR) systems
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- portable: tested on Linux and Windows
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o How to install pg_standby
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$make
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$make install
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o How to use pg_standby?
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pg_standby should be used within the restore_command of the
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recovery.conf file. See the main PostgreSQL manual for details.
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The basic usage should be like this:
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restore_command = 'pg_standby archiveDir %f %p %r'
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with the pg_standby command usage as
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pg_standby [OPTION]... ARCHIVELOCATION NEXTWALFILE XLOGFILEPATH [RESTARTWALFILE]
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When used within the restore_command the %f and %p macros
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will provide the actual file and path required for the restore/recovery.
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pg_standby assumes that ARCHIVELOCATION is directory accessible by the
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server-owning user.
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If RESTARTWALFILE is specified, typically by using the %r option, then all files
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prior to this file will be removed from ARCHIVELOCATION. This then minimises
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the number of files that need to be held, whilst at the same time maintaining
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restart capability. This capability additionally assumes that ARCHIVELOCATION
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directory is writable.
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o options
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pg_standby allows the following command line switches
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-c
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use copy/cp command to restore WAL files from archive
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-d
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debug/logging option.
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-k numfiles
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Cleanup files in the archive so that we maintain no more
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than this many files in the archive. This parameter will
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be silently ignored if RESTARTWALFILE is specified, since
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that specification method is more accurate in determining
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the correct cut-off point in archive.
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You should be wary against setting this number too low,
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since this may mean you cannot restart the standby. This
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is because the last restartpoint marked in the WAL files
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may be many files in the past and can vary considerably.
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This should be set to a value exceeding the number of WAL
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files that can be recovered in 2*checkpoint_timeout seconds,
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according to the value in the warm standby postgresql.conf.
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It is wholly unrelated to the setting of checkpoint_segments
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on either primary or standby.
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Setting numfiles to be zero will disable deletion of files
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from ARCHIVELOCATION.
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If in doubt, use a large value or do not set a value at all.
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If you specify neither RESTARTWALFILE nor -k, then -k 0
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will be assumed, i.e. keep all files in archive.
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Default=0, Min=0
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-l
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use ln command to restore WAL files from archive
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WAL files will remain in archive
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Link is more efficient, but the default is copy to
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allow you to maintain the WAL archive for recovery
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purposes as well as high-availability.
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The default setting is not necessarily recommended,
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consult the main database server manual for discussion.
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This option uses the Windows Vista command mklink
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to provide a file-to-file symbolic link. -l will
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not work on versions of Windows prior to Vista.
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Use the -c option instead.
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see http://en.wikipedia.org/wiki/NTFS_symbolic_link
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-r maxretries
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the maximum number of times to retry the restore command if it
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fails. After each failure, we wait for sleeptime * num_retries
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so that the wait time increases progressively, so by default
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we will wait 5 secs, 10 secs then 15 secs before reporting
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the failure back to the database server. This will be
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interpreted as and end of recovery and the Standby will come
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up fully as a result.
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Default=3, Min=0
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-s sleeptime
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the number of seconds to sleep between testing to see
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if the file to be restored is available in the archive yet.
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The default setting is not necessarily recommended,
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consult the main database server manual for discussion.
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Default=5, Min=1, Max=60
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-t triggerfile
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the presence of the triggerfile will cause recovery to end
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whether or not the next file is available
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It is recommended that you use a structured filename to
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avoid confusion as to which server is being triggered
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when multiple servers exist on same system.
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e.g. /tmp/pgsql.trigger.5432
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-w maxwaittime
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the maximum number of seconds to wait for the next file,
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after which recovery will end and the Standby will come up.
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A setting of zero means wait forever.
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The default setting is not necessarily recommended,
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consult the main database server manual for discussion.
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Default=0, Min=0
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Note: --help is not supported since pg_standby is not intended
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for interactive use, except during dev/test
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o examples
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Linux
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archive_command = 'cp %p ../archive/%f'
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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'
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which will
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- use a ln command to restore WAL files from archive
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- produce logfile output in standby.log
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- keep the last 255 full WAL files, plus the current one
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- sleep for 2 seconds between checks for next WAL file is full
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- never timeout if file not found
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- stop waiting when a trigger file called /tmp.pgsql.trigger.5442 appears
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Windows
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archive_command = 'copy %p ..\\archive\\%f'
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Note that backslashes need to be doubled in the archive_command, but
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*not* in the restore_command, in 8.2, 8.1, 8.0 on Windows.
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restore_command = 'pg_standby -c -d -s 5 -w 0 -t C:\pgsql.trigger.5442 ..\archive %f %p 2>> standby.log'
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which will
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- use a copy command to restore WAL files from archive
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- produce logfile output in standby.log
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- sleep for 5 seconds between checks for next WAL file is full
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- never timeout if file not found
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- stop waiting when a trigger file called C:\pgsql.trigger.5442 appears
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o supported versions
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pg_standby is designed to work with PostgreSQL 8.2 and later. It is
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currently compatible across minor changes between the way 8.3 and 8.2
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operate.
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PostgreSQL 8.3 provides the %r command line substitution, designed to
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let pg_standby know the last file it needs to keep. If the last
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parameter is omitted, no error is generated, allowing pg_standby to
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function correctly with PostgreSQL 8.2 also. With PostgreSQL 8.2,
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the -k option must be used if archive cleanup is required. This option
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remains available in 8.3.
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o reported test success
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SUSE Linux 10.2
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Windows XP Pro
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o additional design notes
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The use of a move command seems like it would be a good idea, but
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this would prevent recovery from being restartable. Also, the last WAL
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file is always requested twice from the archive.
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