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 andPAGE_2CKPT. This information changes for each check-
point. During each checkpoint,tbinit writes the latest checkpoint
informationto one of the pages in the pair. During the next checkpoint, tbinit
writesthe 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
themost-recent checkpoint and the other page in the pair contains a copy of
information written at the second most-recent checkpoint.
Thetbinit daemon follows a different procedure for updating information in
the next three reserved page pairs. Thetbinit daemon only updates the
dbspace, primary chunk, or mirror chunk reserved pages when a change
occurs. Thetbinit 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.
Ifthe flag is set, tbinit prepares to write the modified descriptor information
tothe 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 thePAGE_1CKPT and PAGE_2CKPT pages.)
Thelast pair of reserved pages contains archive information. During a check-
point,tbinit always updates one of the pages in the PAGE_1ARCH/
PAGE_2ARCHreserved-pagepair. The tbinit daemonalternates between each
page in the pair with every checkpoint.
Refer topage 2-72 for a description of the complete checkpoint procedure.