Cloning standbys
Barman
Cloning a standby from Barman
can use
2ndQuadrant's
Barman application
to clone a standby (and also as a fallback source for WAL files).
Barman (aka PgBarman) should be considered as an integral part of any
PostgreSQL replication cluster. For more details see:
https://www.pgbarman.org/.
Barman support provides the following advantages:
the primary node does not need to perform a new backup every time a
new standby is cloned
a standby node can be disconnected for longer periods without losing
the ability to catch up, and without causing accumulation of WAL
files on the primary node
WAL management on the primary becomes much easier as there's no need
to use replication slots, and wal_keep_segments
does not need to be set.
Prerequisites for cloning from Barman
In order to enable Barman support for repmgr standby clone, following
prerequisites must be met:
the barman_server setting in repmgr.conf is the same as the
server configured in Barman;
the barman_host setting in repmgr.conf is set to the SSH
hostname of the Barman server;
the restore_command setting in repmgr.conf is configured to
use a copy of the barman-wal-restore script shipped with the
barman-cli package (see below);
the Barman catalogue includes at least one valid backup for this server.
Barman support is automatically enabled if barman_server
is set. Normally it is good practice to use Barman, for instance
when fetching a base backup while cloning a standby; in any case,
Barman mode can be disabled using the --without-barman
command line option.
If you have a non-default SSH configuration on the Barman
server, e.g. using a port other than 22, then you can set those
parameters in a dedicated Host section in ~/.ssh/config
corresponding to the value ofbarman_host in
repmgr.conf. See the Host
section in man 5 ssh_config for more details.
It's now possible to clone a standby from Barman, e.g.:
NOTICE: using configuration file "/etc/repmgr.conf"
NOTICE: destination directory "/var/lib/postgresql/data" provided
INFO: connecting to Barman server to verify backup for test_cluster
INFO: checking and correcting permissions on existing directory "/var/lib/postgresql/data"
INFO: creating directory "/var/lib/postgresql/data/repmgr"...
INFO: connecting to Barman server to fetch server parameters
INFO: connecting to upstream node
INFO: connected to source node, checking its state
INFO: successfully connected to source node
DETAIL: current installation size is 29 MB
NOTICE: retrieving backup from Barman...
receiving file list ...
(...)
NOTICE: standby clone (from Barman) complete
NOTICE: you can now start your PostgreSQL server
HINT: for example: pg_ctl -D /var/lib/postgresql/data start
Using Barman as a WAL file source
As a fallback in case streaming replication is interrupted, PostgreSQL can optionally
retrieve WAL files from an archive, such as that provided by Barman. This is done by
setting restore_command in recovery.conf to
a valid shell command which can retrieve a specified WAL file from the archive.
barman-wal-restore is a Python script provided as part of the barman-cli
package (Barman 2.0 and later; for Barman 1.x the script is provided separately as
barman-wal-restore.py) which performs this function for Barman.
To use barman-wal-restore with &repmgr;
and assuming Barman is located on the barmansrv host
and that barman-wal-restore is located as an executable at
/usr/bin/barman-wal-restore,
repmgr.conf should include the following lines:
barman_host=barmansrv
barman_server=somedb
restore_command=/usr/bin/barman-wal-restore barmansrv somedb %f %p
barman-wal-restore supports command line switches to
control parallelism (--parallel=N) and compression (
--bzip2, --gzip).
To use a non-default Barman configuration file on the Barman server,
specify this in repmgr.conf with barman_config:
barman_config=/path/to/barman.conf