postgres/doc/src/sgml/diskusage.sgml

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

 <para>
  This chapter discusses how to monitor the disk usage of a
  <productname>PostgreSQL</> 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 is also a <acronym>TOAST</> 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 index on the
   <acronym>TOAST</> table, if present. There might also 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 from three ways:  using
   SQL functions listed in <xref linkend="functions-admin-dbsize">,
   using <command>VACUUM</> information, and from the command line 
   using the tools in <filename>contrib/oid2name</>.  The SQL functions
   are the easiest to use and report information about tables, tables with
   indexes and long value storage (TOAST), databases, and tablespaces.
  </para>

  <para>
   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, <structfield>relpages</>
   is only updated by <command>VACUUM</>, <command>ANALYZE</>, and
   a few DDL commands such as <command>CREATE INDEX</>.)  The
   <structfield>relfilenode</> value is of interest if you want to examine
   the table's disk file directly.
  </para>

  <para>
   To show the space used by <acronym>TOAST</> tables, use a query
   like the following:
<programlisting>
SELECT relname, relpages
    FROM pg_class,
         (SELECT reltoastrelid FROM pg_class
          WHERE relname = 'customer') ss
    WHERE oid = ss.reltoastrelid
       OR oid = (SELECT reltoastidxid FROM pg_class
                 WHERE oid = 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_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>
   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 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 space entirely.
  </para>
 </sect1>
</chapter>