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>56.3. Free Space Map</A
></H1
><P
>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 <TT
CLASS="LITERAL"
>_fsm</TT
> suffix. For example,
if the filenode of a relation is 12345, the FSM is stored in a file called
<TT
CLASS="FILENAME"
>12345_fsm</TT
>, 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 <TT
CLASS="FILENAME"
>src/backend/storage/freespace/README</TT
> for more details on
how the <ACRONYM
CLASS="ACRONYM"
>FSM</ACRONYM
> is structured, and how it's updated and searched.
The <A
HREF="pgfreespacemap.html"
>pg_freespacemap</A
> module
can be used to examine the information stored in free space maps.</P
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