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.

       --config-file=filename
           Use the specified main server configuration file for the target cluster. This affects
           pg_rewind when it uses internally the postgres command for the rewind operation on
           this cluster (when retrieving restore_command with the option -c/--restore-target-wal
           and when forcing a completion of crash recovery).

       --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, postmaster.pid and .DS_Store 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.