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pg_rewind docs: clarify handling of remote servers

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Bruce Momjian 2016-08-01 12:52:22 -04:00
parent 3ebc88e568
commit 878bd9accb
1 changed files with 49 additions and 45 deletions
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doc/src/sgml/ref/pg_rewind.sgml
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@ -16,7 +16,7 @@ PostgreSQL documentation
<refnamediv> <refnamediv>
<refname>pg_rewind</refname> <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> </refnamediv>
<refsynopsisdiv> <refsynopsisdiv>
@ -44,17 +44,18 @@ PostgreSQL documentation
<application>pg_rewind</> is a tool for synchronizing a PostgreSQL cluster <application>pg_rewind</> is a tool for synchronizing a PostgreSQL cluster
with another copy of the same cluster, after the clusters' timelines have 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 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>
<para> <para>
The result is equivalent to replacing the target data directory with the 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 advantage of <application>pg_rewind</> over taking a new base backup, or
tools like <application>rsync</>, is that <application>pg_rewind</> does tools like <application>rsync</>, is that <application>pg_rewind</> does
not require reading through all unchanged files in the cluster. That makes not require reading through unchanged blocks in the cluster. This makes
it a lot faster when the database is large and only a small portion of it it a lot faster when the database is large and only a small
differs between the clusters. fraction of blocks differ between the clusters.
</para> </para>
<para> <para>
@ -62,38 +63,37 @@ PostgreSQL documentation
and target clusters to determine the point where they diverged, and and target clusters to determine the point where they diverged, and
expects to find WAL in the target cluster's <filename>pg_xlog</> directory 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 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 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 shut down soon after the divergence, this is not a problem, but if the
target cluster had run for a long time after the divergence, the old WAL target cluster ran for a long time after the divergence, the old WAL
files might not be present anymore. In that case, they can be manually files might no longer be present. In that case, they can be manually
copied from the WAL archive to the <filename>pg_xlog</> directory. Fetching copied from the WAL archive to the <filename>pg_xlog</> directory, or
missing files from a WAL archive automatically is currently not supported. fetched on startup by configuring <filename>recovery.conf</>. The use of
Besides, <application>pg_rewind</> use cases are not limited by failover. <application>pg_rewind</> is not limited to failover, e.g. a standby
For instance, standby server could be promoted, run some writes and server can be promoted, run some write transactions, and then rewinded
then be returned back as standby. to become a standby again.
</para> </para>
<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 <application>pg_rewind</>, it will go into recovery mode and replay all
WAL generated in the source server after the point of divergence. 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 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</> was run, and therefore could not be copied by the
<application>pg_rewind</> session, it needs to be made available when the <application>pg_rewind</> session, it must be made available when the
target server is started up. That can be done by creating a target server is started. This can be done by creating a
<filename>recovery.conf</> file in the target data directory with a <filename>recovery.conf</> file in the target data directory with a
suitable <varname>restore_command</>. suitable <varname>restore_command</>.
</para> </para>
<para> <para>
<application>pg_rewind</> requires that the target server either has <application>pg_rewind</> requires that the target server either has
the <xref linkend="guc-wal-log-hints"> option is enabled the <xref linkend="guc-wal-log-hints"> option enabled
in <filename>postgresql.conf</> or that data checksums were enabled when in <filename>postgresql.conf</> or data checksums enabled when
the cluster was initialized with <application>initdb</>. Neither of these the cluster was initialized with <application>initdb</>. Neither of these
are currently on by default. are currently on by default. <xref linkend="guc-full-page-writes">
<xref linkend="guc-full-page-writes"> must also be enabled. That is the must also be set to <literal>on</>, but is enabled by default.
default.
</para> </para>
</refsect1> </refsect1>
@ -111,7 +111,7 @@ PostgreSQL documentation
<listitem> <listitem>
<para> <para>
This option specifies the target data directory that is synchronized 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> running <application>pg_rewind</application>
</para> </para>
</listitem> </listitem>
@ -121,9 +121,9 @@ PostgreSQL documentation
<term><option>--source-pgdata=<replaceable class="parameter">directory</replaceable></option></term> <term><option>--source-pgdata=<replaceable class="parameter">directory</replaceable></option></term>
<listitem> <listitem>
<para> <para>
Specifies path to the data directory of the source server, to Specifies the file system path to the data directory of the source
synchronize the target with. This option requires the source server server to synchronize the target with. This option requires the
to be cleanly shut down. source server to be cleanly shut down.
</para> </para>
</listitem> </listitem>
</varlistentry> </varlistentry>
@ -135,8 +135,8 @@ PostgreSQL documentation
Specifies a libpq connection string to connect to the source Specifies a libpq connection string to connect to the source
<productname>PostgreSQL</> server to synchronize the target with. <productname>PostgreSQL</> server to synchronize the target with.
The connection must be a normal (non-replication) connection The connection must be a normal (non-replication) connection
with superuser access. This option requires the server to be running with superuser access. This option requires the source
and not in recovery mode. server to be running and not in recovery mode.
</para> </para>
</listitem> </listitem>
</varlistentry> </varlistentry>
@ -157,7 +157,7 @@ PostgreSQL documentation
<listitem> <listitem>
<para> <para>
Enables progress reporting. Turning this on will deliver an approximate 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> </para>
</listitem> </listitem>
</varlistentry> </varlistentry>
@ -205,38 +205,42 @@ PostgreSQL documentation
<title>How it works</title> <title>How it works</title>
<para> <para>
The basic idea is to copy everything from the new cluster to the old The basic idea is to copy all file system-level changes from the source
cluster, except for the blocks that we know to be the same. cluster to the target cluster:
</para> </para>
<procedure> <procedure>
<step> <step>
<para> <para>
Scan the WAL log of the old cluster, starting from the last checkpoint Scan the WAL log of the target cluster, starting from the last
before the point where the new cluster's timeline history forked off checkpoint before the point where the source cluster's timeline
from the old cluster. For each WAL record, make a note of the data history forked off from the target cluster. For each WAL record,
blocks that were touched. This yields a list of all the data blocks record each data block that was touched. This yields a list of all
that were changed in the old cluster, after the new cluster forked off. the data blocks that were changed in the target cluster, after the
source cluster forked off.
</para> </para>
</step> </step>
<step> <step>
<para> <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> </para>
</step> </step>
<step> <step>
<para> <para>
Copy all other files such as <filename>clog</filename> and configuration files from the new cluster Copy all other files such as <filename>pg_clog</filename> and
to the old cluster, everything except the relation files. configuration files from the source cluster to the target cluster
(everything except the relation files).
</para> </para>
</step> </step>
<step> <step>
<para> <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</> created at failover. (Strictly speaking, <application>pg_rewind</>
doesn't apply the WAL, it just creates a backup label file indicating doesn't apply the WAL, it just creates a backup label file that
that when <productname>PostgreSQL</> is started, it will start replay makes <productname>PostgreSQL</> start by replaying all WAL from
from that checkpoint and apply all the required WAL.) that checkpoint forward.)
</para> </para>
</step> </step>
</procedure> </procedure>