PG_REWIND(1)            PostgreSQL 9.5.3 Documentation            PG_REWIND(1)



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.

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
   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 9.5.3                     2016                         PG_REWIND(1)
