Skip to content

PostgreSQL Monitoring Disk Usage

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

    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 also might be a TOAST file associated with the table, which is used to store values too wide to fit comfortably in the main table. There will be one valid index on the TOAST table, if present. There also might 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.

    You can monitor disk space in three ways: using the SQL functions, using the 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.

    Using psql on a recently vacuumed or analyzed database, you can issue queries to see the disk usage of any table:

    SELECT pg_relation_filepath(oid), relpages FROM pg_class WHERE relname = 'customer';
     pg_relation_filepath | relpages
     base/16384/16806     |       60
    (1 row)

    Each page is typically 8 kilobytes. (Remember, relpages is only updated by VACUUMANALYZE, and a few DDL commands such as CREATE INDEX.) The file path name 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') 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

    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_index |       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

    Disk Full Failure

    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.

    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.

    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.