IBM 000-8697 Blobspace Page Types

2-148 IBM Informix OnLine Database Server Administrator’s Guide
Blobspace Page Types
Blobspace Page Types
Every blobspace chunk contains three types of pages:
■Blobspace free-map page
■Bit-map page (which tracks the blobspace free-map pages)
■Blobpage
Blobspace Free-Map Page
Theblobspace free-map page locates unused blobpages and allocates them as
part of blob creation. When a blobpage is allocated, the free-map entry for
that page is updated. All entries for a single blob are linked.
A blobspace free-map page is the size of one page (specified asBUFFSIZE in
theconfiguration file). Each entry on a free-map page is 8 bytes, stored as two
32-bit words:
■Thefirst bit in the first word specifies whether the blobpage is free or
used.
■The next 31 bits in the first word identify the logical log that was
current when this blobpage was written. (This is needed for
blobpage logging when the logical log file is backed up. Refer to
page 4-26.)
■The second word contains the tblspace number associated with the
blob stored on this page.
The number of entries that can fit on a free-map page depends on the page
size of your machine. The number of free-map pages in a blobspace chunk
depends on the number of blobpages in the chunk.
Blobspace Bit-Map Page
The blobspace bit-map page tracks the fullness and number of blobspace
free-map pages in the chunk. Each blobspace bit-map page is capable of
tracking a quantity of free-map pages that represent more than four million
blobpages.Each blobspace bit-map page is the size of one page (specified as
BUFFSIZE in the configuration file).