PG_REWIND(1) PostgreSQL 9.5.0 Documentation PG_REWIND(1)NAMEpg_rewind - synchronize a PostgreSQL data directory with another data
directory that was forked from the first one
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. All files are copied, 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 all unchanged
files in the cluster. That makes it a lot faster when the database is
large and only a small portion of it differs 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_xlog directory reaching all the way
back to the point of divergence. In the typical failover scenario where
the target cluster was shut down soon after the divergence, that is not
a problem, but if the target cluster had run for a long time after the
divergence, the old WAL files might not be present anymore. In that
case, they can be manually copied from the WAL archive to the pg_xlog
directory. Fetching missing files from a WAL archive automatically is
currently not supported.
When the target server is started up 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 pg_rewind session, it needs to be
made available when the target server is started up. That 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 is enabled in postgresql.conf or that data checksums were
enabled when the cluster was initialized with initdb. Neither of these
are currently on by default. full_page_writes must also be enabled.
That is the 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 shut down
cleanly before running pg_rewind
--source-pgdata=directory
Specifies path to the data directory of the source server, to
synchronize the target with. When --source-pgdata is used, the
source server must 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. The server must be up and running, and must not be 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 over 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 31.14,
“Environment Variables”, in the documentation).
NOTES
How it works
The basic idea is to copy everything from the new cluster to the old
cluster, except for the blocks that we know to be the same.
1. Scan the WAL log of the old cluster, starting from the last
checkpoint before the point where the new cluster's timeline
history forked off from the old cluster. For each WAL record, make
a note of the data blocks that were touched. This yields a list of
all the data blocks that were changed in the old cluster, after the
new cluster forked off.
2. Copy all those changed blocks from the new cluster to the old
cluster.
3. Copy all other files such as clog and configuration files from the
new cluster to the old cluster, everything except the relation
files.
4. Apply the WAL from the new cluster, starting from the checkpoint
created at failover. (Strictly speaking, pg_rewind doesn't apply
the WAL, it just creates a backup label file indicating that when
PostgreSQL is started, it will start replay from that checkpoint
and apply all the required WAL.)
PostgreSQL 9.5.0 2016 PG_REWIND(1)
