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pg_basebackup and replication slots

As the first post dedicated to the feature coverage of Postgres 9.6 which is currently in active development, let’s talk about the following commit:
commit: 0dc848b0314d63188919f1ce943730eac684dccd
author: Peter Eisentraut
date: Tue, 28 Jul 2015 20:31:35 -0400
pg_basebackup: Add --slot option

This option specifies a replication slot for WAL streaming (-X stream),
so that there can be continuous replication slot use between WAL
streaming during the base backup and the start of regular streaming
When taking a base backup with pg_basebackup, one has surely already experienced the problem that WAL files may be missing on the node from where the backup has been taken when connecting for example a fresh standby using this base backup after a certain delay. In such a case the standby node would complain about the following error.
FATAL:  could not receive data from WAL stream:
ERROR:  requested WAL segment 000000010000000000000003 has already been removed
This can be easily avoided by having a WAL archive with a proper restore_command set in the standby’s recovery.conf or by tuning wal_keep_segments with a more or less appropriate number of segments corresponding to the amount of data generated between the moment the base backup has been started and the moment a standby node performing streaming and using this base backup connects to its parent node. In the former case, some users may not have a WAL archive set up (well they normally should to be able to recover from only base backups). In the later case, setting up wal_keep_segments is not an exact science, and if the server faces a peak of activity you may still finish with a missing WAL segments on the original node.
Well, this is where [physical replication slots] (http://www.postgresql.org/docs/devel/static/warm-standby.html#STREAMING-REPLICATION-SLOTS) are actually useful, because once created and enabled for a given client, they are able to retain WAL segments as long at the slot’s restart_lsn is not consumed by this client. Now, combined with pg_basebackup, what you actually get is the possibility to create a base backup and to ensure that WAL segments will be present on the node from where the base backup has been taken when combining it with the stream mode (-X stream). Hence this makes sure that a given base backup does not become useless after having taken it because of missing WAL segments lost until the moment the base backup node is switched on. However be sure to have some kind of monitoring to ensure that pg_xlog on the original node does not get bloated because of the WAL retained, and that the base backup is used becore pg_xlog partition or PGDATA gets full.
In order to use this feature, first create a physical replication slot on the node from which the base backup will be taken:
=# SELECT * FROM pg_create_physical_replication_slot('base_backup_slot');
    slot_name     | xlog_position
 base_backup_slot | null
(1 row)
=# SELECT slot_name, restart_lsn FROM pg_replication_slots;
    slot_name     | restart_lsn
 base_backup_slot | null
(1 row)
Then invoke pg_basebackup with the option –slot and the name of the slot previously created:
$ pg_basebackup -D base_backup --slot base_backup_slot -X stream
Once the base backup has been created the slot is activated and will begin to retain the WAL needed for this base backup.
$ psql -c "SELECT slot_name, restart_lsn FROM pg_replication_slots;"
    slot_name     | restart_lsn
 base_backup_slot | 0/2000000
(1 row)
Note as well that when -R and –slot are used together to generate automatically a recovery.conf file in the base backup, primary_slot_name will be added with the slot name wanted, which is handy as well.


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