PG_REWIND(1) PostgreSQL 14.15 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
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
This option specifies the target data directory that is
synchronized with the source. The target server must be shut down
cleanly before running pg_rewind
--source-pgdata=directory
Specifies the file system path to the data directory of the source
server to synchronize the target with. This option requires the
source server to be cleanly shut down.
--source-server=connstr
Specifies a libpq connection string to connect to the source
PostgreSQL server to synchronize the target with. The connection
must be a normal (non-replication) connection with a role having
sufficient permissions to execute the functions used by pg_rewind
on the source server (see Notes section for details) or a superuser
role. This option requires the source server to be running and
accepting connections.
-R
--write-recovery-conf
Create standby.signal and append connection settings to
postgresql.auto.conf in the output directory. --source-server is
mandatory with this option.
-n
--dry-run
Do everything except actually modifying the target directory.
-N
--no-sync
By default, pg_rewind will wait for all files to be written safely
to disk. This option causes pg_rewind to return without waiting,
which is faster, but means that a subsequent operating system crash
can leave the data directory corrupt. Generally, this option is
useful for testing but should not be used on a production
installation.
-P
--progress
Enables progress reporting. Turning this on will deliver an
approximate progress report while copying data from the source
cluster.
-c
--restore-target-wal
Use restore_command defined in the target cluster configuration to
retrieve WAL files from the WAL archive if these files are no
longer available in the pg_wal directory.
--debug
Print verbose debugging output that is mostly useful for developers
debugging pg_rewind.
--no-ensure-shutdown
pg_rewind requires that the target server is cleanly shut down
before rewinding. By default, if the target server is not shut down
cleanly, pg_rewind starts the target server in single-user mode to
complete crash recovery first, and stops it. By passing this
option, pg_rewind skips this and errors out immediately if the
server is not cleanly shut down. Users are expected to handle the
situation themselves in that case.
-V
--version
Display version information, then exit.
-?
--help
Show help, then exit.
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;
When executing pg_rewind using an online cluster as source which has
been recently promoted, it is necessary to execute a CHECKPOINT after
promotion such that its control file reflects up-to-date timeline
information, which is used by pg_rewind to check if the target cluster
can be rewound using the designated source cluster.
How It Works
The basic idea is to copy all file system-level changes from the source
cluster to the target cluster:
1. Scan the WAL log of the target cluster, starting from the last
checkpoint before the point where the source cluster's timeline
history forked off from the target cluster. For each WAL record,
record each data block that was touched. This yields a list of all
the data blocks that were changed in the target cluster, after the
source cluster forked off. If some of the WAL files are no longer
available, try re-running pg_rewind with the -c option to search
for the missing files in the WAL archive.
2. Copy all those changed blocks from the source cluster to the target
cluster, either using direct file system access (--source-pgdata)
or SQL (--source-server). Relation files are now in a state
equivalent to the moment of the last completed checkpoint prior to
the point at which the WAL timelines of the source and target
diverged plus the current state on the source of any blocks changed
on the target after that divergence.
3. Copy all other files, including new relation files, WAL segments,
pg_xact, and configuration files from the source cluster to the
target cluster. Similarly to base backups, the contents of the
directories pg_dynshmem/, pg_notify/, pg_replslot/, pg_serial/,
pg_snapshots/, pg_stat_tmp/, and pg_subtrans/ are omitted from the
data copied from the source cluster. The files backup_label,
tablespace_map, pg_internal.init, postmaster.opts, and
postmaster.pid, as well as any file or directory beginning with
pgsql_tmp, are omitted.
4. Create a backup_label file to begin WAL replay at the checkpoint
created at failover and configure the pg_control file with a
minimum consistency LSN defined as the result of
pg_current_wal_insert_lsn() when rewinding from a live source or
the last checkpoint LSN when rewinding from a stopped source.
5. When starting the target, PostgreSQL replays all the required WAL,
resulting in a data directory in a consistent state.
PostgreSQL 14.15 2024 PG_REWIND(1)
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