pg_rewind - Online in the Cloud

PROGRAM:

NAME

pg_rewind - synchronize a PostgreSQL data directory with another data directory that was
forked from the first one

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. 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.

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 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.)

Use pg_rewind online using onworks.net services