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<!--
$Header: /cvsroot/pgsql/doc/src/sgml/diskusage.sgml,v 1.9 2003/03/24 14:32:50 petere Exp $
-->
<chapter id="diskusage">
<title>Monitoring Disk Usage</title>
<para>
This chapter discusses how to monitor the disk usage of a
<productname>PostgreSQL</> database system. In the current
release, the database administrator does not have much control over
the on-disk storage layout, so this chapter is mostly informative
and can give you some ideas how to manage the disk usage with
operating system tools.
</para>
<sect1 id="disk-usage">
<title>Determining Disk Usage</Title>
<indexterm zone="disk-usage">
<primary>disk usage</primary>
</indexterm>
<para>
Each table has a primary heap disk file where most of the data is
stored. To store long column values, there is also a
<acronym>TOAST</> file associated with the table, named based on the
table's OID (actually <literal>pg_class.relfilenode</>), and an index on the
<acronym>TOAST</> table. There also may be indexes associated with
the base table.
</para>
<para>
You can monitor disk space from three places: from
<application>psql</> using <command>VACUUM</> information, from
<application>psql</> using the tools in <filename>contrib/dbsize</>, and from
the command line using the tools in <filename>contrib/oid2name</>. Using
<application>psql</> on a recently vacuumed or analyzed database,
you can issue queries to see the disk usage of any table:
<programlisting>
SELECT relfilenode, relpages FROM pg_class WHERE relname = 'customer';
relfilenode | relpages
-------------+----------
16806 | 60
(1 row)
</programlisting>
Each page is typically 8 kilobytes. (Remember, <literal>relpages</>
is only updated by <command>VACUUM</> and <command>ANALYZE</>.)
</para>
<para>
To show the space used by <acronym>TOAST</> tables, use a query
like the following, substituting the <literal>relfilenode</literal>
number of the heap (determined by the query above):
<programlisting>
SELECT relname, relpages
FROM pg_class
WHERE relname = 'pg_toast_16806' OR relname = 'pg_toast_16806_index'
ORDER BY relname;
relname | relpages
----------------------+----------
pg_toast_16806 | 0
pg_toast_16806_index | 1
</programlisting>
</para>
<para>
You can easily display index sizes, too:
<programlisting>
SELECT c2.relname, c2.relpages
FROM pg_class c, pg_class c2, pg_index i
WHERE c.relname = 'customer'
AND c.oid = i.indrelid
AND c2.oid = i.indexrelid
ORDER BY c2.relname;
relname | relpages
----------------------+----------
customer_id_indexdex | 26
</programlisting>
</para>
<para>
It is easy to find your largest tables and indexes using this
information:
<programlisting>
SELECT relname, relpages FROM pg_class ORDER BY relpages DESC;
relname | relpages
----------------------+----------
bigtable | 3290
customer | 3144
</programlisting>
</para>
<para>
<filename>contrib/dbsize</> loads functions into your database that allow
you to find the size of a table or database from inside
<application>psql</> without the need for <command>VACUUM</> or <command>ANALYZE</>.
</para>
<para>
You can also use <filename>contrib/oid2name</> to show disk usage. See
<filename>README.oid2name</> in that directory for examples. It includes a script that
shows disk usage for each database.
</para>
</sect1>
<sect1 id="disk-full">
<title>Disk Full Failure</title>
<para>
The most important disk monitoring task of a database administrator
is to make sure the disk doesn't grow full. A filled data disk may
result in subsequent corruption of database indexes, but not of the
tables themselves. If the WAL files are on the same disk (as
is the case for a default configuration) then a filled disk during
database initialization may result in corrupted or incomplete WAL
files. This failure condition is detected and the database server
will refuse to start up.
</para>
<para>
If you cannot free up additional space on the disk by deleting
other things you can move some of the database files to other file
systems and create a symlink from the original location. But
note that <application>pg_dump</> cannot save the location layout
information of such a setup; a restore would put everything back in
one place. To avoid running out of disk space, you can place the
WAL files or individual databases in other locations while creating
them. See the <command>initdb</> documentation and <xref
linkend="manage-ag-alternate-locs"> for more information about that.
</para>
<tip>
<para>
Some file systems perform badly when they are almost full, so do
not wait until the disk is full to take action.
</para>
</tip>
</sect1>
</chapter>
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