Provided by: postgresql-11_11.5-1_amd64 bug


       pg_rewind - synchronize a PostgreSQL data directory with another data directory that was
       forked from it


       pg_rewind [option...] {-D  | --target-pgdata} directory {--source-pgdata=directory |


       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, or fetched on startup by
       configuring recovery.conf. The use of pg_rewind is not limited to failover, e.g. a standby
       server can be promoted, run some write transactions, and then rewinded to become a standby

       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.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 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.

           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

           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.


       pg_rewind accepts the following command-line arguments:

       -D 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

           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

           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.

           Do everything except actually modifying the target directory.

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

           Print verbose debugging output that is mostly useful for developers debugging

           Display version information, then exit.

           Show help, then exit.


       When --source-server option is used, pg_rewind also uses the environment variables
       supported by libpq (see Section 34.14).


       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

        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.)