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pg_rewind(1) - phpMan
PG_REWIND(1) PostgreSQL 12.3 Documentation PG_REWIND(1)
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, or fetched on startup by
configuring primary_conninfo or restore_command. 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 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 synchronized 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.)
PostgreSQL 12.3 2020 PG_REWIND(1)
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