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PG_REWIND(1) | PostgreSQL 16.4 Documentation | PG_REWIND(1) |
NAME¶
pg_rewind - synchronize a PostgreSQL data directory with another data directory that was forked from it
SYNOPSIS¶
pg_rewind [option...] {-D | --target-pgdata} directory {--source-pgdata=directory | --source-server=connstr}
DESCRIPTION¶
pg_rewind is a tool for synchronizing a PostgreSQL cluster with another copy of the same cluster, after the clusters' timelines have diverged. A typical scenario is to bring an old primary server back online after failover as a standby that follows the new primary.
After a successful rewind, the state of the target data directory is analogous to a base backup of the source data directory. Unlike taking a new base backup or using a tool like rsync, pg_rewind does not require comparing or copying unchanged relation blocks in the cluster. Only changed blocks from existing relation files are copied; all other files, including new relation files, configuration files, and WAL segments, are copied in full. As such the rewind operation is significantly faster than other approaches when the database is large and only a small fraction of blocks differ between the clusters.
pg_rewind examines the timeline histories of the source and target clusters to determine the point where they diverged, and expects to find WAL in the target cluster's pg_wal directory reaching all the way back to the point of divergence. The point of divergence can be found either on the target timeline, the source timeline, or their common ancestor. In the typical failover scenario where the target cluster was shut down soon after the divergence, this is not a problem, but if the target cluster ran for a long time after the divergence, its old WAL files might no longer be present. In this case, you can manually copy them from the WAL archive to the pg_wal directory, or run pg_rewind with the -c option to automatically retrieve them from the WAL archive. The use of pg_rewind is not limited to failover, e.g., a standby server can be promoted, run some write transactions, and then rewound to become a standby again.
After running pg_rewind, WAL replay needs to complete for the data directory to be in a consistent state. When the target server is started again it will enter archive recovery and replay all WAL generated in the source server from the last checkpoint before the point of divergence. If some of the WAL was no longer available in the source server when pg_rewind was run, and therefore could not be copied by the pg_rewind session, it must be made available when the target server is started. This can be done by creating a recovery.signal file in the target data directory and by configuring a suitable restore_command in postgresql.conf.
pg_rewind requires that the target server either has the wal_log_hints option enabled in postgresql.conf or data checksums enabled when the cluster was initialized with initdb. Neither of these are currently on by default. full_page_writes must also be set to on, but is enabled by default.
Warning: Failures While Rewinding
If pg_rewind fails while processing, then the data folder of the target is likely not in a state that can be recovered. In such a case, taking a new fresh backup is recommended.
As pg_rewind copies configuration files entirely from the source, it may be required to correct the configuration used for recovery before restarting the target server, especially if the target is reintroduced as a standby of the source. If you restart the server after the rewind operation has finished but without configuring recovery, the target may again diverge from the primary.
pg_rewind will fail immediately if it finds files it cannot write directly to. This can happen for example when the source and the target server use the same file mapping for read-only SSL keys and certificates. If such files are present on the target server it is recommended to remove them before running pg_rewind. After doing the rewind, some of those files may have been copied from the source, in which case it may be necessary to remove the data copied and restore back the set of links used before the rewind.
OPTIONS¶
pg_rewind accepts the following command-line arguments:
-D directory
--target-pgdata=directory
--source-pgdata=directory
--source-server=connstr
-R
--write-recovery-conf
-n
--dry-run
-N
--no-sync
-P
--progress
-c
--restore-target-wal
--config-file=filename
--debug
--no-ensure-shutdown
-V
--version
-?
--help
ENVIRONMENT¶
When --source-server option is used, pg_rewind also uses the environment variables supported by libpq (see Section 34.15).
The environment variable PG_COLOR specifies whether to use color in diagnostic messages. Possible values are always, auto and never.
NOTES¶
When executing pg_rewind using an online cluster as source, a role having sufficient permissions to execute the functions used by pg_rewind on the source cluster can be used instead of a superuser. Here is how to create such a role, named rewind_user here:
CREATE USER rewind_user LOGIN; GRANT EXECUTE ON function pg_catalog.pg_ls_dir(text, boolean, boolean) TO rewind_user; GRANT EXECUTE ON function pg_catalog.pg_stat_file(text, boolean) TO rewind_user; GRANT EXECUTE ON function pg_catalog.pg_read_binary_file(text) TO rewind_user; GRANT EXECUTE ON function pg_catalog.pg_read_binary_file(text, bigint, bigint, boolean) TO rewind_user;
How It Works¶
The basic idea is to copy all file system-level changes from the source cluster to the target cluster:
2024 | PostgreSQL 16.4 |