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- Each heap and index relation, except for hash indexes, has a Free Space Map
- (FSM) to keep track of available space in the relation. It's stored
- alongside the main relation data in a separate relation fork, named after the
- filenode number of the relation, plus a <code class="literal">_fsm</code> suffix. For example,
- if the filenode of a relation is 12345, the FSM is stored in a file called
- <code class="filename">12345_fsm</code>, in the same directory as the main relation file.
- </p><p>
- The Free Space Map is organized as a tree of <acronym class="acronym">FSM</acronym> pages. The
- bottom level <acronym class="acronym">FSM</acronym> pages store the free space available on each
- heap (or index) page, using one byte to represent each such page. The upper
- levels aggregate information from the lower levels.
- </p><p>
- Within each <acronym class="acronym">FSM</acronym> page is a binary tree, stored in an array with
- one byte per node. Each leaf node represents a heap page, or a lower level
- <acronym class="acronym">FSM</acronym> page. In each non-leaf node, the higher of its children's
- values is stored. The maximum value in the leaf nodes is therefore stored
- at the root.
- </p><p>
- See <code class="filename">src/backend/storage/freespace/README</code> for more details on
- how the <acronym class="acronym">FSM</acronym> is structured, and how it's updated and searched.
- The <a class="xref" href="pgfreespacemap.html" title="F.26. pg_freespacemap">pg_freespacemap</a> module
- can be used to examine the information stored in free space maps.
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