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Update High Availability docs. Clarify terms master/primary standby/slave, 

move two paragraphs that apply to log shipping in general from the
"Alternative method for log shipping" section to the earlier sections.
Add varname tags where missing. Some small wording changes.
This commit is contained in:
Heikki Linnakangas 2010-05-28 14:03:31 +00:00
parent f39d57b83c
commit fe76f93d11
1 changed files with 57 additions and 53 deletions
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110
doc/src/sgml/high-availability.sgml
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@ -1,4 +1,4 @@
<!-- $PostgreSQL: pgsql/doc/src/sgml/high-availability.sgml,v 1.68 2010/05/13 14:16:41 mha Exp $ -->
<!-- $PostgreSQL: pgsql/doc/src/sgml/high-availability.sgml,v 1.69 2010/05/28 14:03:31 heikki Exp $ -->
<chapter id="high-availability">
<title>High Availability, Load Balancing, and Replication</title>
@ -38,10 +38,12 @@
<para>
Some solutions deal with synchronization by allowing only one
server to modify the data. Servers that can modify data are
called read/write or "master" servers. Servers that can reply
to read-only queries are called "slave" servers. Servers that
cannot be accessed until they are changed to master servers are
called "standby" servers.
called read/write, <firstterm>master</> or <firstterm>primary</> servers.
Servers that track changes in the master are called <firstterm>standby</>
or <firstterm>slave</> servers. A standby server that cannot be connected
to until it is promoted to a master server is called a <firstterm>warm
standby</> server, and one that can accept connections and serves read-only
queries is called a <firstterm>hot standby</> server.
</para>
<para>
@ -144,16 +146,16 @@ protocol to make nodes agree on a serializable transactional order.
<para>
Warm and hot standby servers can be kept current by reading a
stream of write-ahead log (<acronym>WAL</>)
records. If the main server fails, the warm standby contains
records. If the main server fails, the standby contains
almost all of the data of the main server, and can be quickly
made the new master database server. This is asynchronous and
can only be done for the entire database server.
</para>
<para>
A PITR standby server can be kept more up-to-date using streaming
replication.; see <xref linkend="streaming-replication">. For
warm standby information, see <xref linkend="warm-standby">, and
for hot standby, see <xref linkend="hot-standby">.
A PITR standby server can be implemented using file-based log shipping
(<xref linkend="warm-standby">) or streaming replication (see
<xref linkend="streaming-replication">), or a combination of both. For
information on hot standby, see <xref linkend="hot-standby">.
</para>
</listitem>
</varlistentry>
@ -613,10 +615,9 @@ protocol to make nodes agree on a serializable transactional order.
<para>
Standby mode is exited and the server switches to normal operation,
when a trigger file is found (trigger_file). Before failover, it will
restore any WAL available in the archive or in pg_xlog, but won't try
to connect to the master or wait for files to become available in the
archive.
when a trigger file is found (<varname>trigger_file</>). Before failover,
any WAL immediately available in the archive or in pg_xlog will be
restored, but no attempt is made to connect to the master.
</para>
</sect2>
@ -710,6 +711,15 @@ trigger_file = '/path/to/trigger_file'
replication, make sure you set <varname>max_wal_senders</> high enough in
the primary to allow them to be connected simultaneously.
</para>
<para>
If you're using a WAL archive, its size can be minimized using
the <varname>restartpoint_command</> option to remove files that are no
no longer required by the standby server. Note however, that if you're
using the archive for backup purposes, you need to retain files needed
to recover from at least the latest base backup, even if they're no
longer needed by the standby.
</para>
</sect2>
<sect2 id="streaming-replication">
@ -737,15 +747,14 @@ trigger_file = '/path/to/trigger_file'
</para>
<para>
Streaming replication relies on file-based continuous archiving for
making the base backup and for allowing the standby to catch up if it is
disconnected from the primary for long enough for the primary to
delete old WAL files still required by the standby. It is possible
to use streaming replication without WAL archiving, but if a standby
falls behind too much, the primary will delete old WAL files still
needed by the standby, and the standby will have to be manually restored
from a base backup. You can control how long the primary retains old WAL
segments using the <varname>wal_keep_segments</> setting.
If you use streaming replication without file-based continuous
archiving, you have to set <varname>wal_keep_segments</> in the master
to a value high enough to ensure that old WAL segments are not recycled
too early, while the standby might still need them to catch up. If the
standby falls behind too much, it needs to be reinitialized from a new
base backup. If you set up a WAL archive that's accessible from the
standby, wal_keep_segments is not required as the standby can always
use the archive to catch up.
</para>
<para>
@ -881,6 +890,14 @@ primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass'
rigorous testing.
</para>
<para>
<productname>PostgreSQL</productname> does not provide the system
software required to identify a failure on the primary and notify
the standby database server. Many such tools exist and are well
integrated with the operating system facilities required for
successful failover, such as IP address migration.
</para>
<para>
Once failover to the standby occurs, there is only a
single server in operation. This is known as a degenerate state.
@ -921,7 +938,7 @@ primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass'
<para>
An alternative to the built-in standby mode described in the previous
sections is to use a restore_command that polls the archive location.
sections is to use a <varname>restore_command</> that polls the archive location.
This was the only option available in versions 8.4 and below. In this
setup, set <varname>standby_mode</> off, because you are implementing
the polling required for standby operation yourself. See
@ -932,9 +949,9 @@ primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass'
<para>
Note that in this mode, the server will apply WAL one file at a
time, so if you use the standby server for queries (see Hot Standby),
there is a bigger delay between an action in the master and when the
there is a delay between an action in the master and when the
action becomes visible in the standby, corresponding the time it takes
to fill up the WAL file. archive_timeout can be used to make that delay
to fill up the WAL file. <varname>archive_timeout</> can be used to make that delay
shorter. Also note that you can't combine streaming replication with
this method.
</para>
@ -991,14 +1008,6 @@ if (!triggered)
configurations and environments.
</para>
<para>
<productname>PostgreSQL</productname> does not provide the system
software required to identify a failure on the primary and notify
the standby database server. Many such tools exist and are well
integrated with the operating system facilities required for
successful failover, such as IP address migration.
</para>
<para>
The method for triggering failover is an important part of planning
and design. One potential option is the <varname>restore_command</>
@ -1015,20 +1024,12 @@ if (!triggered)
creation of a trigger file is ideal, if this can be arranged.
</para>
<para>
The size of the WAL archive can be minimized by using the <literal>%r</>
option of the <varname>restore_command</>. This option specifies the
last archive file name that needs to be kept to allow the recovery to
restart correctly. This can be used to truncate the archive once
files are no longer required, assuming the archive is writable from the
standby server.
</para>
<sect2 id="warm-standby-config">
<title>Implementation</title>
<para>
The short procedure for configuring a standby server is as follows. For
The short procedure for configuring a standby server using this alternative
method is as follows. For
full details of each step, refer to previous sections as noted.
<orderedlist>
<listitem>
@ -1086,9 +1087,10 @@ if (!triggered)
<title>Record-based Log Shipping</title>
<para>
<productname>PostgreSQL</productname> directly supports file-based
log shipping as described above. It is also possible to implement
record-based log shipping, though this requires custom development.
It is also possible to implement record-based log shipping using this
alternative method, though this requires custom development, and changes
will still only becomes visible to hot standby queries after a full WAL
file has been shipped.
</para>
<para>
@ -1573,9 +1575,9 @@ if (!triggered)
<para>
Three-way deadlocks are possible between <literal>AccessExclusiveLocks</> arriving from
the primary, cleanup WAL records that require buffer cleanup locks, and
user requests that are waiting behind replayed <literal>AccessExclusiveLocks</>. Deadlocks
are resolved immediately, should they occur, though they are thought to be
rare in practice.
user requests that are waiting behind replayed <literal>AccessExclusiveLocks</>.
Deadlocks are resolved automatically after <varname>deadlock_timeout</>
seconds, though they are thought to be rare in practice.
</para>
<para>
@ -1684,8 +1686,10 @@ LOG: database system is ready to accept read only connections
themselves. Users will still write large sort temporary files and
re-generate relcache info files, so no part of the database
is truly read-only during hot standby mode.
Note also that writes to remote databases will still be possible,
even though the transaction is read-only locally.
Note also that writes to remote databases using
<application>dblink</application> module, and other operations outside the
database using PL functions will still be possible, even though the
transaction is read-only locally.
</para>
<para>
@ -1940,7 +1944,7 @@ LOG: database system is ready to accept read only connections
</indexterm>
<para>
In a warm standby configuration, it is possible to offload the expense of
In a standby configuration, it is possible to offload the expense of
taking periodic base backups from the primary server; instead base backups
can be made by backing
up a standby server's files. This concept is generally known as