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.)