IBM 000-8697 Structure of a Dbspace Bit-Map Page, 4-Bit Bit-Mapped Pages

2-144 IBM Informix OnLine Database Server Administrator’s Guide
Structure of a Dbspace Bit-Map Page
Two bits are all that are needed to describe page fullness for these limited
conditions, as illustrated here.
4-Bit Bit-Mapped Pages
The 4-bit bit-map pages track available space in extents allocated to tables
thatcontain rows longer than a page, or rows that include VARCHAR,BYTE,
or TEXT data types. Four bits are needed to describe all possible combina-
tions of page fullness for these extents, as illustrated below. The terms used
to describe page fullness describe row segments aswhole-page,partial-page,
and small. These segment sizes are relative to available free space and are
selected on the basis of performance.
Bit Values Description of Page Fullness
00 Page is unused
10 Page is used completely (index page)
01 Page is partially used (data page)
11 Page is full (data page)
Bit Values Description of Page Fullness
0000 Page is unused
0100 Home data page has room for another data row
1000 Page is used completely (index page)
1100 Home data page is full
0001 Remainder page, can accept whole-page segments
0101 Remainder page, room for partial-page segments
1001 Remainder page, room for small segments
1101 Remainder page, no room for even small segments
0010 Blob page, can accept whole-page segments
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