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System Architecture 2-143
Structure of a Dbspace Bit-Map Page
Structure of a Dbspace Bit-Map Page
Extents contain one or more bit-map pages that track free pages in the extent.
Each bit-map entry describes the fullness of one page in the extent. The
number of bit-map pages needed for an extent depends on three variables:
■ Number of pages in the extent, which affects the number of bit-map
entries needed
■ Page size, which affects the number of bit-map entries that can fit on
a page
■ Type of the bit-map entries, which depends on the type of data stored
on the page
All bit-map pages are initialized and linked when the extent is allocated. The
bit-map pages are scattered throughout the extent. The first page in the
extent, and every (n +1)th page thereafter, is designated as a bit-map page,
where n is the number of bit-map entries that fit on a single page. The pages
described by a bit-map page can span extents.
OnLine uses two types of bit-map pages, a 2-bit bit-map page (which
contains 2-bit entries) and a 4-bit bit-map page (which contains 4-bit entries).
2-Bit Bit-Mapped Pages
The 2-bit bit-map pages track available space in extents allocated to tables
that meet two criteria:
■ The table contains fixed-length rows that are smaller than a page.
■ The table does not contain VARCHAR, BYTE, or TEXT data types.