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Chapter 24. Monitoring Disk Usage

Table of Contents
24.1. Determining Disk Usage
24.2. Disk Full Failure

This chapter discusses how to monitor the disk usage of a PostgreSQL database system.

24.1. Determining Disk Usage

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 TOAST file associated with the table, which is used to store values too wide to fit comfortably in the main table (see Section 49.2). There will be one index on the TOAST table, if present. There may also be indexes associated with the base table. Each table and index is stored in a separate disk file — possibly more than one file, if the file would exceed one gigabyte. Naming conventions for these files are described in Section 49.1.

You can monitor disk space from three places: from psql using VACUUM information, from psql using the tools in contrib/dbsize, and from the command line using the tools in contrib/oid2name. Using psql on a recently vacuumed or analyzed database, you can issue queries to see the disk usage of any table:

SELECT relfilenode, relpages FROM pg_class WHERE relname = 'customer';

 relfilenode | relpages 
       16806 |       60
(1 row)

Each page is typically 8 kilobytes. (Remember, relpages is only updated by VACUUM, ANALYZE, and a few DDL commands such as CREATE INDEX.) The relfilenode value is of interest if you want to examine the table's disk file directly.

To show the space used by TOAST tables, use a query like the following:

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

You can easily display index sizes, too:

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

It is easy to find your largest tables and indexes using this information:

SELECT relname, relpages FROM pg_class ORDER BY relpages DESC;

       relname        | relpages 
 bigtable             |     3290
 customer             |     3144

contrib/dbsize loads functions into your database that allow you to find the size of a table or database from inside psql without the need for VACUUM or ANALYZE.

You can also use contrib/oid2name to show disk usage. See README.oid2name in that directory for examples. It includes a script that shows disk usage for each database.