PG_REWIND(1) PostgreSQL 10.1 Documentation PG_REWIND(1)NAMEpg_rewind - synchronize a PostgreSQL data directory with another data
directory that was forked from it
SYNOPSISpg_rewind [option...] {-D | --target-pgdata} directory
{--source-pgdata=directory | --source-server=connstr}
DESCRIPTIONpg_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 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.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.
OPTIONSpg_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 superuser
access. 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.
-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).
NOTES
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).
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 10.1 2017 PG_REWIND(1)
