2-96 IBM Informix OnLine Database Server Administrator’s Guide
Reserved Pages
The reserved page checkpoint information is stored in a two-page pair,
PAGE_1CKPT and PAGE_2CKPT. This information changes for each check-
point. During each checkpoint, tbinit writes the latest checkpoint
information to one of the pages in the pair. During the next checkpoint, tbinit
writes the information to the other page in the pair. At any point, one page in
the checkpoint reserved page pair contains a copy of information written at
the most-recent checkpoint and the other page in the pair contains a copy of
information written at the second most-recent checkpoint.
The tbinit daemon follows a different procedure for updating information in
the next three reserved page pairs. The tbinit daemon only updates the
dbspace, primary chunk, or mirror chunk reserved pages when a change
occurs. The tbinit daemon learns of a change from flags that are set on the
dbspace, primary chunk, and mirror descriptor tables in shared memory.
During the checkpoint, tbinit checks each shared-memory descriptor table
for a change flag.
If the flag is set, tbinit prepares to write the modified descriptor information
to the appropriate page in the reserved page pair. First, tbinit switches from
the current page (which is the page that received the last write) to the other
page in the pair. Second, tbinit writes the information to the reserved page.
Third, tbinit updates the fields that contain the numbers of the current pages
for the dbspace, primary chunk, or mirror chunk information. (These fields
are located on the PAGE_1CKPT and PAGE_2CKPT pages.)
The last pair of reserved pages contains archive information. During a check-
point, tbinit always updates one of the pages in the PAGE_1ARCH/
PAGE_2ARCH
reserved-pagepair. The tbinitdaemon alternatesbetween each
page in the pair with every checkpoint.
Refer to page 2-72 for a description of the complete checkpoint procedure.
