IBM 000-8697 Manual Do Utilizador

Data Consistency, Recovery, and Migration

(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 to

The checkpoint timestamp becomes the standard against which disk pages
are compared for archiving purposes. (Timestamps are not based on system
time. Refer to

 for further information about timestamps.)

For example, if the checkpoint occurs at 3401, then for a level-0 archive, all
pages containing timestamps less than 3401 must be copied to tape. As tbtape
reads through disk pages during the archive, pages with timestamps greater
than 3401 are ignored. OnLine relies on the logical log files to contain records
of modifications that occur after 3401.

During the checkpoint, tbtape also creates a snapshot of information that is
needed as reference to execute the archive procedure.

The snapshot information describes, for every OnLine chunk, the pages that
were allocated 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.

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 does tbtape overcome the
problem of archiving every page at its 3401-state if OnLine processing is
constantly modifying pages?