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4.4.2 Free Space Map

These parameters control the size of the shared free space map, which tracks the locations of unused space in the database. An undersized free space map may cause the database to consume increasing amounts of disk space over time, because free space that is not in the map cannot be re-used; instead PostgreSQL will request more disk space from the operating system when it needs to store new data. The last few lines displayed by a database-wide VACUUM VERBOSE command can help in determining if the current settings are adequate. A NOTICE message is also printed during such an operation if the current settings are too low.

Increasing these parameters may cause PostgreSQL to request more System V shared memory than your operating system's default configuration allows. See section 3.4.1 Shared Memory and Semaphores for information on how to adjust those parameters, if necessary.

max_fsm_pages (integer)

Sets the maximum number of disk pages for which free space will be tracked in the shared free-space map. Six bytes of shared memory are consumed for each page slot. This setting must be at least 16 * max_fsm_relations. The default is chosen by initdb depending on the amount of available memory, and can range from 20k to 200k pages. This parameter can only be set at server start.

max_fsm_relations (integer)

Sets the maximum number of relations (tables and indexes) for which free space will be tracked in the shared free-space map. Roughly seventy bytes of shared memory are consumed for each slot. The default is one thousand relations. This parameter can only be set at server start.