Description

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 25.2).

To configure a standby server to use pg_standby, put this into its postgresql.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:

Options

pg_standby accepts the following command-line arguments:

Notes

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

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:

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:

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 Also