PostgreSQL version:

wal_keep_segments

Specifies the minimum number of past log file segments kept in the pg_wal 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 sending server falls behind by more than wal_keep_segments segments, the sending server might remove a WAL segment still needed by the standby, in which case the replication connection will be terminated. Downstream connections will also eventually fail as a result. (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_wal; 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, so 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 can only be set in the postgresql.conf file or on the server command line.

Recommendations

… if using replication. Minimum number of WAL log segments to keep in order to support re-synchronizing streaming standby servers which have fallen behind or for an initial sync, a good rule of thumb is 4, or however many segments you go through in 30s, whichever is higher. This is in addition to max_wal_size, so make sure you have enough disk space. Not required if you are archiving logs.

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