PG_STANDBY(1) PostgreSQL 10.1 Documentation PG_STANDBY(1)NAMEpg_standby - supports the creation of a PostgreSQL warm standby server
SYNOPSISpg_standby [option...] archivelocation nextwalfile walfilepath
[restartwalfile]
DESCRIPTIONpg_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:
Smart Failover
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.
Fast Failover
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.
OPTIONSpg_standby accepts the following command-line arguments:
-c
Use cp or copy command to restore WAL files from archive. This is
the only supported behavior so this option is useless.
-d
Print lots of debug logging output on stderr.
-k
Remove 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 maxretries
Set 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 sleeptime
Set 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 triggerfile
Specify 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
--version
Print the pg_standby version and exit.
-w maxwaittime
Set 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.
-?
--help
Show help about pg_standby command line arguments, and exit.
NOTESpg_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
EXAMPLES
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.
AUTHOR
Simon Riggs <simon@2ndquadrant.com>
SEE ALSOpg_archivecleanup(1)PostgreSQL 10.1 2017 PG_STANDBY(1)
