IBM 000-8697 Manual de usuario descargar pdf (Pagina 237)

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System Architecture 2-143

Structure of a Dbspace Bit-Map Page

Extentscontainoneormorebit-mappagesthattrackfreepagesintheextent.

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 numberof bit-map entries thatcan ﬁt on

a page

■ Typeofthebit-mapentries,whichdependsonthetypeofdatastored

on the page

Allbit-map pages are initialized and linkedwhentheextentis 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

contains2-bitentries) and a 4-bitbit-mappage(whichcontains4-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.

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