In this article, we look at the parameters related to PostgreSQL replication.
The figure below shows the parameters that need to be set for each type of replication. Each parameter is described below in alphabetical order under Replication Parameter.
Replication Parameter
archive_cleanup_command
Used to clean up archived WAL files that the standby server no longer needs. The pg_archivecleanup application is commonly used to make WAL file cleanup easy.
ㅤ | Description |
Applies to | Standby Server |
Category | WRITE-AHEAD LOG → Archive Recovery |
Restart required | Not required |
Changes | • This parameter was introduced in PostgreSQL 12. |
archive_command
Specifies the shell command run to store a WAL file in the archive directory when the file fills up. The setting below means: copy the WAL file in the pg_wal directory to the /PostgreSQL/Archive directory.
ㅤ | Description |
Applies to | Main Server |
Category | WRITE-AHEAD LOG → Archiving |
Restart required | Not required |
Placeholders | • %p : absolute path of the WAL file
• %f : archive file name |
archive_mode
When archive mode is enabled, WAL files stored in the WAL directory (pg_wal) are passed to the archive directory through the command set in
archive_command (usually copied). Because the oldest WAL files are deleted according to the wal_keep_size parameter (wal_keep_segments in PostgreSQL 12 and earlier), archive mode is used to keep the WAL files that get deleted. It must be set to wal_level = replica or higher; with wal_level = minimal archive mode cannot be used.ㅤ | Description |
Applies to | Main Server |
Category | WRITE-AHEAD LOG → Archiving |
Default | off |
Restart required | Required |
Values | always, on, off |
archive_timeout
Normally a WAL file switches to a new WAL file only when it fills up, so when there is nothing to write to the WAL file, for example when there are no transactions, no archiving happens. Without archiving, the data lag between the main server and the standby server lasts longer in file-based replication and streaming replication setups. The
archive_timeout parameter sets the interval at which WAL files are switched periodically, which makes archiving happen periodically. It is usually set to about 60 seconds; setting it too short makes the archive directory grow, so be careful.ㅤ | Description |
Applies to | Main Server |
Category | WRITE-AHEAD LOG → Archiving |
Default | 0 |
Restart required | Not required |
hot_standby
Must be set to on on a standby server where the database is in standby state and replication is running. If set to off, you cannot run SQL while replication is in progress.
ㅤ | Description |
Applies to | Standby Server |
Category | WRITE-AHEAD LOG → Standby Servers |
Default | on |
Restart required | Required |
Values | on, off |
max_replication_slots
Specifies the maximum number of replication slots the database can support. The default is 10, and if you set it lower than the number of replication slots that currently exist, the server will not start. To use replication slots, it must also be set to
wal_level = replica or higher.ㅤ | Description |
Applies to | Main Server |
Category | REPLICATION → Sending Servers |
Default | 10 (0 in PostgreSQL 9.6 and earlier) |
Restart required | Required |
Changes | • Starting with PostgreSQL 10, the default changed from 0 to 10. |
max_wal_senders
Specifies the maximum number of connections to WAL sender processes. The default is 10. The number of WAL sender processes cannot be set higher than
max_connections, and it is usually set to the number of standby servers + 1.ㅤ | Description |
Applies to | Main Server |
Category | REPLICATION → Sending Servers |
Default | 10 (0 up to PostgreSQL 9.6) |
Restart required | Required |
Changes | • Starting with PostgreSQL 10, the default changed from 0 to 10. (0 means replication is disabled.) |
primary_conninfo
Sets the connection string used to connect to the main server. It typically specifies the main server's host (IP), port, a user with the replication privilege, and the password.
ㅤ | Description |
Applies to | Standby Server |
Category | WRITE-AHEAD LOG → Standby Servers |
syntax | primary_conninfo = ‘host=10.10.45.230 port=5432 user=replicauser password=mypass’ |
Restart required | Not required |
Changes | • This parameter was introduced in PostgreSQL 12.
• Up to PostgreSQL 11, it was set in recovery.conf. |
primary_slot_name
Specifies the name of the replication slot used on the main server. If you do not use replication slots, you do not need to set it.
ㅤ | Description |
Applies to | Standby Server |
Category | WRITE-AHEAD LOG → Standby Servers |
Restart required | Not required |
Changes | • This parameter was introduced in PostgreSQL 12.
• Up to PostgreSQL 11, it was set in recovery.conf. |
restore_command
Specifies the shell command used to restore the database from WAL archive files.
ㅤ | Description |
Applies to | Standby Server |
Category | WRITE-AHEAD LOG → Archive Recovery |
Restart required | Not required |
Changes | • This parameter was introduced in PostgreSQL 12.• Up to PostgreSQL 11, it was set in recovery.conf. |
Placeholders | • %p : absolute path to copy the WAL file to
• %f : archive file name |
wal_keep_size
Specifies the total size of WAL files that can be kept in the pg_wal directory, where WAL files are stored. It is set in megabytes (MB). A single WAL file is normally 16MB, so you can calculate the number of WAL files from
wal_keep_size. Adjust the number of WAL files to prevent replication from stopping because WAL files were deleted before the standby server replicated them.ㅤ | Description |
Applies to | Main Server |
Category | REPLICATION → Sending Servers |
Default | 0 |
Restart required | Not required |
Changes | • This parameter was introduced in PostgreSQL 13.
• Up to PostgreSQL 12, the wal_keep_segments parameter, which specifies the number of WAL files, was used. |
wal_level
Introduced in PostgreSQL 9.0, this parameter determines how much information is written to WAL. Possible values are minimal, replica, and logical. Set it to replica to use physical replication, and to logical to use logical replication.
ㅤ | Description |
Applies to | Main Server |
Location | WRITE-AHEAD LOG → Settings |
Default | replica (minimal in PostgreSQL 9.6 and earlier) |
Restart required | Required |
Values | • minimal : writes only the information needed to recover from a crash or immediate shutdown.
• replica : merges archive and hot_standby starting with PostgreSQL 9.6.
- archive : minimal + adds the information needed for WAL archiving.
- hot_standby : archive + the information needed to run read-only queries on a standby server
• logical : adds the information needed to support replica + logical decoding. More data is stored, so WAL grows larger. |
Changes | • logical was added in PostgreSQL 9.4.
• Starting with PostgreSQL 9.6, replica replaces archive and hot_standby.
• Starting with PostgreSQL 10, the default changed from minimal to replica. |
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