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<!--
$Header: /cvsroot/pgsql/doc/src/sgml/diskusage.sgml,v 1.3 2002/06/21 19:17:40 momjian Exp $
-->
<chapter id="diskusage">
<title>Monitoring Disk Usage</title>
<sect1 id="disk-usage">
<title>Monitoring 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 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 <application>contrib/dbsize</>, and from
the command line using <application>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>
play=# SELECT relfilenode, relpages
play-# FROM pg_class
play-# WHERE relname = 'customer';
relfilenode | relpages
-------------+----------
16806 | 60
(1 row)
</programlisting>
</para>
<para>
Each page is typically 8 kilobytes. (Remember, <literal>relpages</>
is only updated by <command>VACUUM</> and <command>ANALYZE</>.) To
show the space used by <acronym>TOAST</> tables, use a query based on
the heap relfilenode shown above:
<programlisting>
play=# SELECT relname, relpages
play-# FROM pg_class
play-# WHERE relname = 'pg_toast_16806' or
play-# relname = 'pg_toast_16806_index'
play-# ORDER BY relname;
relname | relpages
----------------------+----------
pg_toast_16806 | 0
pg_toast_16806_index | 1
</programlisting>
</para>
<para>
You can easily display index usage too:
<programlisting>
play=# SELECT c2.relname, c2.relpages
play-# FROM pg_class c, pg_class c2, pg_index i
play-# WHERE c.relname = 'customer' AND
play-# c.oid = i.indrelid AND
play-# c2.oid = i.indexrelid
play-# ORDER BY c2.relname;
relname | relpages
----------------------+----------
customer_id_indexdex | 26
</programlisting>
</para>
<para>
It is easy to find your largest files using <application>psql</>:
<programlisting>
play=# SELECT relname, relpages
play-# FROM pg_class
play-# ORDER BY relpages DESC;
relname | relpages
----------------------+----------
bigtable | 3290
customer | 3144
</programlisting>
</para>
<para>
<application>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/ANALYZE.</>
</para>
<para>
You can also use <application>oid2name</> to show disk usage. See
<filename>README.oid2name</> for examples. It includes a script
shows disk usage for each database.
</para>
</sect1>
</chapter>
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