repmgr standby clone 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
(PostgreSQL 13 and later: wal_keep_size)
does not need to be set.
Currently repmgr's support for cloning from Barman is implemented by using rsync to clone from the Barman server.
It is therefore not able to make use of Barman's parallel restore facility, which is executed on the Barman server and clones to the target server.
Barman's parallel restore facility can be used by executing it manually on
the Barman server and configuring replication on the resulting cloned
standby using
repmgr standby clone --replication-conf-only.
In order to enable Barman support for repmgr standby clone, following
prerequisites must be met:
the Barman catalogue must include at least one valid backup for this server;
the barman_host setting in repmgr.conf is set to the SSH
hostname of the Barman server;
the barman_server setting in repmgr.conf is the same as the
server configured in Barman.
For example, assuming Barman is located on the host "barmansrv"
under the "barman" user account,
repmgr.conf should contain the following entries:
barman_host='barman@barmansrv'
barman_server='pg'
Here pg corresponds to a section in Barman's configuration file for a specific
server backup configuration, which would look something like:
[pg]
description = "Main cluster"
...
More details on Barman configuration can be found in the Barman documentation's configuration section.
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'
We also recommend configuring the restore_command setting in repmgr.conf
to use the barman-wal-restore script
(see section Using Barman as a WAL file source below).
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 of barman_host in
repmgr.conf. See the Host
section in man 5 ssh_config for more details.
If you wish to place WAL files in a location outside the main
PostgreSQL data directory, set --waldir
(PostgreSQL 9.6 and earlier: --xlogdir) in
pg_basebackup_options to the target directory
(must be an absolute filepath). repmgr will create and
symlink to this directory in exactly the same way
pg_basebackup would.
It's now possible to clone a standby from Barman, e.g.:
$ repmgr -f /etc/repmgr.conf -h node1 -U repmgr -d repmgr standby clone
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 source node
DETAIL: current installation size is 30 MB
NOTICE: retrieving backup from Barman...
(...)
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
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.
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 ~ 2.7) or as part of the core Barman distribution (Barman 2.8 and later).
To use barman-wal-restore with repmgr,
assuming Barman is located on the host "barmansrv"
under the "barman" user account,
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='barman@barmansrv'
barman_server='pg'
restore_command='/usr/bin/barman-wal-restore barmansrv pg %f %p'
barman-wal-restore supports command line switches to
control parallelism (--parallel=N) and compression
(--bzip2, --gzip).