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Editing: pg_rewind.1

'\" t
.\"     Title: pg_rewind
.\"    Author: The PostgreSQL Global Development Group
.\" Generator: DocBook XSL Stylesheets v1.79.1 <http://docbook.sf.net/>
.\"      Date: 2018
.\"    Manual: PostgreSQL 9.6.9 Documentation
.\"    Source: PostgreSQL 9.6.9
.\"  Language: English
.\"
.TH "PG_REWIND" "1" "2018" "PostgreSQL 9.6.9" "PostgreSQL 9.6.9 Documentation"
.\" -----------------------------------------------------------------
.\" * Define some portability stuff
.\" -----------------------------------------------------------------
.\" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.\" http://bugs.debian.org/507673
.\" http://lists.gnu.org/archive/html/groff/2009-02/msg00013.html
.\" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.ie \n(.g .ds Aq \(aq
.el       .ds Aq '
.\" -----------------------------------------------------------------
.\" * set default formatting
.\" -----------------------------------------------------------------
.\" disable hyphenation
.nh
.\" disable justification (adjust text to left margin only)
.ad l
.\" -----------------------------------------------------------------
.\" * MAIN CONTENT STARTS HERE *
.\" -----------------------------------------------------------------
.SH "NAME"
pg_rewind \- synchronize a PostgreSQL data directory with another data directory that was forked from it
.SH "SYNOPSIS"
.HP \w'\fBpg_rewind\fR\ 'u
\fBpg_rewind\fR [\fIoption\fR...] {\fB\-D\ \fR | \fB\-\-target\-pgdata\fR}\fI directory\fR {\fB\-\-source\-pgdata=\fR\fB\fIdirectory\fR\fR | \fB\-\-source\-server=\fR\fB\fIconnstr\fR\fR}
.SH "DESCRIPTION"
.PP
pg_rewind
is a tool for synchronizing a PostgreSQL cluster with another copy of the same cluster, after the clusters\*(Aq 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\&.
.PP
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\&.
.PP
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\*(Aqs
pg_xlog
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_xlog
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\&.
.PP
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
\fIrestore_command\fR\&.
.PP
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\&.
.SH "OPTIONS"
.PP
pg_rewind
accepts the following command\-line arguments:
.PP
\fB\-D \fR\fB\fIdirectory\fR\fR
.br
\fB\-\-target\-pgdata=\fR\fB\fIdirectory\fR\fR
.RS 4
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
.RE
.PP
\fB\-\-source\-pgdata=\fR\fB\fIdirectory\fR\fR
.RS 4
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\&.
.RE
.PP
\fB\-\-source\-server=\fR\fB\fIconnstr\fR\fR
.RS 4
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\&.
.RE
.PP
\fB\-n\fR
.br
\fB\-\-dry\-run\fR
.RS 4
Do everything except actually modifying the target directory\&.
.RE
.PP
\fB\-P\fR
.br
\fB\-\-progress\fR
.RS 4
Enables progress reporting\&. Turning this on will deliver an approximate progress report while copying data from the source cluster\&.
.RE
.PP
\fB\-\-debug\fR
.RS 4
Print verbose debugging output that is mostly useful for developers debugging
pg_rewind\&.
.RE
.PP
\fB\-V\fR
.br
\fB\-\-version\fR
.RS 4
Display version information, then exit\&.
.RE
.PP
\fB\-?\fR
.br
\fB\-\-help\fR
.RS 4
Show help, then exit\&.
.RE
.SH "ENVIRONMENT"
.PP
When
\fB\-\-source\-server\fR
option is used,
pg_rewind
also uses the environment variables supported by
libpq
(see
Section 32.14, \(lqEnvironment Variables\(rq, in the documentation)\&.
.SH "NOTES"
.SS "How it works"
.PP
The basic idea is to copy all file system\-level changes from the source cluster to the target cluster:
.sp
.RS 4
.ie n \{\
\h'-04' 1.\h'+01'\c
.\}
.el \{\
.sp -1
.IP "  1." 4.2
.\}
Scan the WAL log of the target cluster, starting from the last checkpoint before the point where the source cluster\*(Aqs 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\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04' 2.\h'+01'\c
.\}
.el \{\
.sp -1
.IP "  2." 4.2
.\}
Copy all those changed blocks from the source cluster to the target cluster, either using direct file system access (\fB\-\-source\-pgdata\fR) or SQL (\fB\-\-source\-server\fR)\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04' 3.\h'+01'\c
.\}
.el \{\
.sp -1
.IP "  3." 4.2
.\}
Copy all other files such as
pg_clog
and configuration files from the source cluster to the target cluster (everything except the relation files)\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04' 4.\h'+01'\c
.\}
.el \{\
.sp -1
.IP "  4." 4.2
.\}
Apply the WAL from the source cluster, starting from the checkpoint created at failover\&. (Strictly speaking,
pg_rewind
doesn\*(Aqt apply the WAL, it just creates a backup label file that makes
PostgreSQL
start by replaying all WAL from that checkpoint forward\&.)
.RE

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