pg_standby - Online in the Cloud

PROGRAM:

NAME

pg_standby - supports the creation of a PostgreSQL warm standby server

SYNOPSIS

pg_standby [option...] archivelocation nextwalfile xlogfilepath [restartwalfile]

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, “Log-Shipping
Standby Servers”, in the documentation).

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.

OPTIONS

pg_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 25.2, “Log-Shipping Standby Servers”, in the
documentation 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 25.2, “Log-Shipping
Standby Servers”, in the documentation for discussion.

-?
--help
Show help about pg_standby command line arguments, and exit.

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:

· 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.

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