<refnamediv>
<refname>pg_rewind</refname>
- <refpurpose>synchronize a <productname>PostgreSQL</productname> data directory with another data directory that was forked from the first one</refpurpose>
+ <refpurpose>synchronize a <productname>PostgreSQL</productname> data directory with another data directory that was forked from it</refpurpose>
</refnamediv>
<refsynopsisdiv>
<application>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.
+ after failover as a standby that follows the new master.
</para>
<para>
The result is equivalent to replacing the target data directory with the
- source one. All files are copied, including configuration files. The
+ source one. Only changed blocks from relation files are copied;
+ all other files are copied in full, including configuration files. The
advantage of <application>pg_rewind</> over taking a new base backup, or
tools like <application>rsync</>, is that <application>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.
+ 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.
</para>
<para>
and target clusters to determine the point where they diverged, and
expects to find WAL in the target cluster's <filename>pg_xlog</> directory
reaching all the way back to the point of divergence. The point of divergence
- could be found either on target timeline, source timeline or their common
+ 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, 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 <filename>pg_xlog</> directory. Fetching
- missing files from a WAL archive automatically is currently not supported.
- Besides, <application>pg_rewind</> use cases are not limited by failover.
- For instance, standby server could be promoted, run some writes and
- then be returned back as standby.
+ 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 <filename>pg_xlog</> directory, or
+ fetched on startup by configuring <filename>recovery.conf</>. The use of
+ <application>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.
</para>
<para>
- When the target server is started up for the first time after running
+ When the target server is started for the first time after running
<application>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
- <application>pg_rewind</> was run, and therefore could not be copied by
- <application>pg_rewind</> session, it needs to be made available when the
- target server is started up. That can be done by creating a
+ <application>pg_rewind</> was run, and therefore could not be copied by the
+ <application>pg_rewind</> session, it must be made available when the
+ target server is started. This can be done by creating a
<filename>recovery.conf</> file in the target data directory with a
suitable <varname>restore_command</>.
</para>
<para>
<application>pg_rewind</> requires that the target server either has
- the <xref linkend="guc-wal-log-hints"> option is enabled
- in <filename>postgresql.conf</> or that data checksums were enabled when
+ the <xref linkend="guc-wal-log-hints"> option enabled
+ in <filename>postgresql.conf</> or data checksums enabled when
the cluster was initialized with <application>initdb</>. Neither of these
- are currently on by default.
- <xref linkend="guc-full-page-writes"> must also be enabled. That is the
- default.
+ are currently on by default. <xref linkend="guc-full-page-writes">
+ must also be set to <literal>on</>, but is enabled by default.
</para>
</refsect1>
<listitem>
<para>
This option specifies the target data directory that is synchronized
- with the source. The target server must shut down cleanly before
+ with the source. The target server must be shut down cleanly before
running <application>pg_rewind</application>
</para>
</listitem>
<term><option>--source-pgdata=<replaceable class="parameter">directory</replaceable></option></term>
<listitem>
<para>
- Specifies 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.
+ 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.
</para>
</listitem>
</varlistentry>
Specifies a libpq connection string to connect to the source
<productname>PostgreSQL</> server to synchronize the target with.
The connection must be a normal (non-replication) connection
- with superuser access. This option requires the server to be running
- and not in recovery mode.
+ with superuser access. This option requires the source
+ server to be running and not in recovery mode.
</para>
</listitem>
</varlistentry>
<listitem>
<para>
Enables progress reporting. Turning this on will deliver an approximate
- progress report while copying data over from the source cluster.
+ progress report while copying data from the source cluster.
</para>
</listitem>
</varlistentry>
<title>How it works</title>
<para>
- 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.
+ The basic idea is to copy all file system-level changes from the source
+ cluster to the target cluster:
</para>
<procedure>
<step>
<para>
- 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.
+ 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.
</para>
</step>
<step>
<para>
- Copy all those changed blocks from the new cluster to the old cluster.
+ Copy all those changed blocks from the source cluster to
+ the target cluster, either using direct file system access
+ (<option>--source-pgdata</>) or SQL (<option>--source-server</>).
</para>
</step>
<step>
<para>
- Copy all other files such as <filename>clog</filename> and configuration files from the new cluster
- to the old cluster, everything except the relation files.
+ Copy all other files such as <filename>pg_clog</filename> and
+ configuration files from the source cluster to the target cluster
+ (everything except the relation files).
</para>
</step>
<step>
<para>
- Apply the WAL from the new cluster, starting from the checkpoint
+ Apply the WAL from the source cluster, starting from the checkpoint
created at failover. (Strictly speaking, <application>pg_rewind</>
- doesn't apply the WAL, it just creates a backup label file indicating
- that when <productname>PostgreSQL</> is started, it will start replay
- from that checkpoint and apply all the required WAL.)
+ doesn't apply the WAL, it just creates a backup label file that
+ makes <productname>PostgreSQL</> start by replaying all WAL from
+ that checkpoint forward.)
</para>
</step>
</procedure>
projects / postgresql / commitdiff