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 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. Only changed blocks
       from relation files are copied; all other files are copied in full, 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 unchanged blocks in the cluster. This makes it a lot faster 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, the old WAL files might no longer be present. In that case, they can be manually
       copied from the WAL archive to the pg_wal directory. 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.

       When the target server is started 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
       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 configuring 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.

           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
           not in recovery mode.

       -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 when creating a production installation.

       -P
       --progress
           Enables progress reporting. Turning this on will deliver an approximate progress report while copying
           data 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 33.14).

       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 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 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.

        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).

        3. Copy all other files such as pg_xact and configuration files from the source cluster to the target
           cluster (everything except the relation files). 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. Any file or directory
           beginning with pgsql_tmp is omitted, as well as are backup_label, tablespace_map, pg_internal.init,
           postmaster.opts and postmaster.pid.

        4. Apply the WAL from the source cluster, starting from the checkpoint created at failover. (Strictly
           speaking, pg_rewind doesn't apply the WAL, it just creates a backup label file that makes PostgreSQL
           start by replaying all WAL from that checkpoint forward.)