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

 <para>
  This chapter discusses how to monitor the disk usage of a 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 <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>

 <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
   fundamental data tables. 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 <application>initdb</> documentation and <xref
   linkend="manage-ag-alternate-locs"> for more information.
  </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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