NAME

       pg_rewind - synchronize a PostgreSQL data directory with another data directory that was forked from the
       first one

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 master server back online
       after failover, as a standby that follows the new master.

       The result is equivalent to replacing the target data directory with the source one. All files are
       copied, including configuration files. The advantage of pg_rewind over taking a new base backup, or tools
       like rsync, is that pg_rewind does not require reading through all unchanged files in the cluster. That
       makes it a lot faster when the database is large and only a small portion of it differs 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_xlog directory reaching all the way
       back to the point of divergence. In the typical failover scenario where the target cluster was shut down
       soon after the divergence, that is not a problem, but if the target cluster had run for a long time after
       the divergence, the old WAL files might not be present anymore. In that case, they can be manually copied
       from the WAL archive to the pg_xlog directory. Fetching missing files from a WAL archive automatically is
       currently not supported.

       When the target server is started up for the first time after running pg_rewind, it will go into recovery
       mode and replay all WAL generated in the source server after 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
       pg_rewind session, it needs to be made available when the target server is started up. That can be done
       by creating a recovery.conf file in the target data directory with a suitable restore_command.

       pg_rewind requires that the target server either has the wal_log_hints option is enabled in
       postgresql.conf or that data checksums were enabled when the cluster was initialized with initdb. Neither
       of these are currently on by default.  full_page_writes must also be enabled. That is the 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.

           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 shut down cleanly before running pg_rewind

       --source-pgdata=directory
           Specifies path to the data directory of the source server, to synchronize the target with. When
           --source-pgdata is used, the source server must 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 superuser access. The
           server must be up and running, and must not be in recovery mode.

       -n
       --dry-run
           Do everything except actually modifying the target directory.

       -P
       --progress
           Enables progress reporting. Turning this on will deliver an approximate progress report while copying
           data over from the source cluster.

       --debug
           Print verbose debugging output that is mostly useful for developers debugging pg_rewind.

       -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 31.14, “Environment Variables”, in the documentation).

NOTES

       When executing pg_rewind using an online cluster as source which has been recently promoted, it is
       necessary to execute a CHECKPOINT after promotion so as 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 everything from the new cluster to the old cluster, except for the blocks that
       we know to be the same.

        1. Scan the WAL log of the old cluster, starting from the last checkpoint before the point where the new
           cluster's timeline history forked off from the old cluster. For each WAL record, make a note of the
           data blocks that were touched. This yields a list of all the data blocks that were changed in the old
           cluster, after the new cluster forked off.

        2. Copy all those changed blocks from the new cluster to the old cluster.

        3. Copy all other files such as clog and configuration files from the new cluster to the old cluster,
           everything except the relation files.

        4. Apply the WAL from the new cluster, starting from the checkpoint created at failover. (Strictly
           speaking, pg_rewind doesn't apply the WAL, it just creates a backup label file indicating that when
           PostgreSQL is started, it will start replay from that checkpoint and apply all the required WAL.)

PostgreSQL 9.5.25                                     2021                                          PG_REWIND(1)