<!ENTITY backup SYSTEM "backup.sgml">

<!ENTITY charset SYSTEM "charset.sgml">

<!ENTITY client-auth SYSTEM "client-auth.sgml">

<!ENTITY diskusage SYSTEM "diskusage.sgml">

<!ENTITY high-availability SYSTEM "high-availability.sgml">

<!ENTITY installbin SYSTEM "install-binaries.sgml">

<!ENTITY installation SYSTEM "installation.sgml">

</sect1>

<sect1 id="diskusage">

<title>Monitoring Disk Usage</title>

<para>

This section discusses how to monitor the disk usage of a

<productname>PostgreSQL</productname> database system.

</para>

<sect2 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>

</sect2>

<sect2 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>

</sect2>

</sect1>

</chapter>

&backup;

&high-availability;

&monitoring;

&diskusage;

&wal;

&logical-replication;

&jit;