Data Consistency, Recovery, and Migration 4-33

Synchronize tbtape and tbinit Activities

(It is likely that some transactions are ongoing during an online archive
procedure. The restore procedure describes how transactions that span the
archive tape and the logical log file are rolled back during a data restore, if
necessary. Refer topage 4-45.)

Purpose of Checkpoint Timestamp

The checkpoint timestamp becomes the standard against which disk pages
arecompared for archiving purposes. (Timestamps are not based on system
time. Refer topage 2-44 for further information about timestamps.)
For example, if the checkpoint occurs at 3401, then for a level-0 archive, all
pagescontaining timestamps less than 3401 must be copied to tape. As tbtape
readsthrough disk pages during the archive, pages with timestamps greater
than3401 are ignored. OnLine relies on the logical log files to contain records
of modifications that occur after 3401.

Purpose of Data Snapshot

During the checkpoint,tbtape also creates a snapshot of information that is
needed as reference to execute the archiveprocedure.
Thesnapshot information describes, for every OnLine chunk, the pages that
wereallocated and the pages that were free at the time of the checkpoint. This
information is needed because the status of the pages might change during
an online archive. OnLine does not read pages that were considered free at
the time of the begin-archive checkpoint.
The snapshot also defines the pages that composed the logical log files and
the physical log files at the time of the checkpoint. This snapshot enables
tbtape to recognize and skip (not archive) pages that were allocated to logs
at the time of the checkpoint.
Synchronize tbtape and tbinit Activities
During an online archive,tbtape archives disk pages at the same time that
OnLine processing modifies disk pages. Assume that the archive-begin
checkpoint for a level-0 occurred at 3401. How doestbtape overcome the
problem of archiving every page at its 3401-state if OnLine processing is
constantly modifying pages?