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4.5.4 Streaming Replication

These settings control the behavior of the built-in streaming replication feature. These parameters would be set on the primary server that is to send replication data to one or more standby servers.

max_wal_senders (integer)

Specifies the maximum number of concurrent connections from standby servers (i.e., the maximum number of simultaneously running WAL sender processes). The default is zero. This parameter can only be set at server start. wal_level must be set to archive or hot_standby to allow connections from standby servers.

wal_sender_delay (integer)

Specifies the delay between activity rounds for WAL sender processes. In each round the WAL sender sends any WAL accumulated since the last round to the standby server. It then sleeps for wal_sender_delay milliseconds, and repeats. The default value is 200 milliseconds (200ms). Note that on many systems, the effective resolution of sleep delays is 10 milliseconds; setting wal_sender_delay to a value that is not a multiple of 10 might have the same results as setting it to the next higher multiple of 10. This parameter can only be set in the ‘postgresql.conf’ file or on the server command line.

wal_keep_segments (integer)

Specifies the minimum number of past log file segments kept in the ‘pg_xlog’ directory, in case a standby server needs to fetch them for streaming replication. Each segment is normally 16 megabytes. If a standby server connected to the primary falls behind by more than wal_keep_segments segments, the primary might remove a WAL segment still needed by the standby, in which case the replication connection will be terminated. (However, the standby server can recover by fetching the segment from archive, if WAL archiving is in use.) This sets only the minimum number of segments retained in ‘pg_xlog’; the system might need to retain more segments for WAL archival or to recover from a checkpoint. If wal_keep_segments is zero (the default), the system doesn't keep any extra segments for standby purposes, and the number of old WAL segments available to standby servers is a function of the location of the previous checkpoint and status of WAL archiving. This parameter has no effect on restartpoints. This parameter can only be set in the ‘postgresql.conf’ file or on the server command line.

vacuum_defer_cleanup_age (integer)

Specifies the number of transactions by which VACUUM and HOT updates will defer cleanup of dead row versions. The default is zero transactions, meaning that dead row versions can be removed as soon as possible, that is, as soon as they are no longer visible to any open transaction. You may wish to set this to a non-zero value on a primary server that is supporting hot standby servers, as described in section 11.5 Hot Standby. This allows more time for queries on the standby to complete without incurring conflicts due to early cleanup of rows. However, since the value is measured in terms of number of write transactions occurring on the primary server, it is difficult to predict just how much additional grace time will be made available to standby queries. This parameter can only be set in the ‘postgresql.conf’ file or on the server command line.