postgres/doc/src/sgml/diskusage.sgml

<!-- doc/src/sgml/diskusage.sgml -->

<chapter id="diskusage">
 <title>Monitoring Disk Usage</title>

 <para>
  This chapter discusses how to monitor the disk usage of a
  <productname>PostgreSQL</productname> database system.
 </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. If the table has any columns with potentially-wide values,
   there also might be a <acronym>TOAST</acronym> file associated with the table,
   which is used to store values too wide to fit comfortably in the main
   table (see <xref linkend="storage-toast"/>).  There will be one valid index
   on the <acronym>TOAST</acronym> table, if present. There also might be indexes
   associated with the base table.  Each table and index is stored in a
   separate disk file &mdash; possibly more than one file, if the file would
   exceed one gigabyte.  Naming conventions for these files are described
   in <xref linkend="storage-file-layout"/>.
  </para>

  <para>
   You can monitor disk space in three ways:
   using the SQL functions listed in <xref linkend="functions-admin-dbsize"/>,
   using the <xref linkend="oid2name"/> module, or
   using manual inspection of the system catalogs.
   The SQL functions are the easiest to use and are generally recommended.
   The remainder of this section shows how to do it by inspection of the
   system catalogs.
  </para>

  <para>
   Using <application>psql</application> on a recently vacuumed or analyzed database,
   you can issue queries to see the disk usage of any table:
<programlisting>
SELECT pg_relation_filepath(oid), relpages FROM pg_class WHERE relname = 'customer';

 pg_relation_filepath | relpages
----------------------+----------
 base/16384/16806     |       60
(1 row)
</programlisting>
   Each page is typically 8 kilobytes. (Remember, <structfield>relpages</structfield>
   is only updated by <command>VACUUM</command>, <command>ANALYZE</command>, and
   a few DDL commands such as <command>CREATE INDEX</command>.)  The file path name
   is of interest if you want to examine the table's disk file directly.
  </para>

  <para>
   To show the space used by <acronym>TOAST</acronym> tables, use a query
   like the following:
<programlisting>
SELECT relname, relpages
FROM pg_class,
     (SELECT reltoastrelid
      FROM pg_class
      WHERE relname = 'customer') AS ss
WHERE oid = ss.reltoastrelid OR
      oid = (SELECT indexrelid
             FROM pg_index
             WHERE indrelid = ss.reltoastrelid)
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_index |       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>
 </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 become full.  A filled data disk will
   not result in data corruption, but it might prevent useful activity
   from occurring. If the disk holding the WAL files grows full, database
   server panic and consequent shutdown might occur.
  </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 by making use of tablespaces. See <xref
   linkend="manage-ag-tablespaces"/> 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 completely full to take action.
   </para>
  </tip>

  <para>
   If your system supports per-user disk quotas, then the database
   will naturally be subject to whatever quota is placed on the user
   the server runs as.  Exceeding the quota will have the same bad
   effects as running out of disk space entirely.
  </para>
 </sect1>
</chapter>