pg_start_backup and pg_stop_backup, the two low-level functions of PostgreSQL that can be used to take a base backup from an instance, have been extended with a new option allowing to take what is called non-exclusive backups. This feature is introduced in PostgreSQL 9.6 by the following commit:
commit: 7117685461af50f50c03f43e6a622284c8d54694 author: Magnus Hagander <email@example.com> date: Tue, 5 Apr 2016 20:03:49 +0200 Implement backup API functions for non-exclusive backups Previously non-exclusive backups had to be done using the replication protocol and pg_basebackup. With this commit it's now possible to make them using pg_start_backup/pg_stop_backup as well, as long as the backup program can maintain a persistent connection to the database. Doing this, backup_label and tablespace_map are returned as results from pg_stop_backup() instead of being written to the data directory. This makes the server safe from a crash during an ongoing backup, which can be a problem with exclusive backups. The old syntax of the functions remain and work exactly as before, but since the new syntax is safer this should eventually be deprecated and removed. Only reference documentation is included. The main section on backup still needs to be rewritten to cover this, but since that is already scheduled for a separate large rewrite, it's not included in this patch. Reviewed by David Steele and Amit Kapila
The existing functions pg_start_backup and pg_stop_backup that are present for ages in Postgres have a couple of limitations that have always been disturbing for some users:
- It is not possible to take multiple backups in parallel.
- In case of a crash of the tool taking the backup, the server remains stuck in backup mode and needs some cleanup actions.
- The backup_label file being created in the data folder, it is not possible to make the difference between a server that crashed while a backup is taken and a cluster restored from a backup.
Some users are able to live with those problems, the application layer in charge of the backups can take up extra cleanup actions in case of a backup tool crash letting the cluster in an inconsistent state, or has a design that assumes that no more than one backup can be taken.
Non-exclusive backups work in such a way that the backup_label file and the tablespace map file are not created in the data folder but are returned as results of pg_stop_backup. In this case the backup tool is the one in charge of writing both files in the backup taken. This has the advantage to leverage all the problems that exclusive backups induce, at the cost of a couple of things though:
- The backup utility is in charge of doing some extra work to put the resulting base backup in a consistent state.
- The connection to the backend needs to remain while the base backup is being taken. If the client disconnects while the backup is taken, it is aborted.
So, in order to control that, a third argument has been added to pg_start_backup. Its default value is true, meaning that an exclusive backup is taken, protecting all the existing backup tools:
=# SELECT pg_start_backup('my_backup', true, false); pg_start_backup ----------------- 0/4000028 (1 row)
Note also that pg_stop_backup uses now an extra argument to track if it needs to stop an exclusive or a non-exclusive backup. With the backup started previously, trying to stop an exclusive backup results in an error:
=# SELECT pg_stop_backup(true); ERROR: 55000: non-exclusive backup in progress HINT: did you mean to use pg_stop_backup('f')? LOCATION: pg_stop_backup_v2, xlogfuncs.c:230
Then let’s stop it correctly, and the resulting fields are what is needed to complete the backup.:
=# SELECT * FROM pg_stop_backup(false); NOTICE: 00000: pg_stop_backup complete, all required WAL segments have been archived LOCATION: do_pg_stop_backup, xlog.c:10569 lsn | labelfile | spcmapfile -----------+---------------------------------------------------------------+------------ 0/4000130 | START WAL LOCATION: 0/4000028 (file 000000010000000000000004)+| | CHECKPOINT LOCATION: 0/4000060 +| | BACKUP METHOD: streamed +| | BACKUP FROM: master +| | START TIME: 2016-05-31 10:34:46 JST +| | LABEL: my_backup +| | | (1 row)
The contents of “labelfile” need to be written as backup_label in the backup taken while the contents of “spcmapfile” need to be written to tablespace_map. Once the contents of those files is written, don’t forget as well to flush them to disk to prevent any potential loss caused by power failures for example.