3-46 IBM Informix OnLine Database Server Administrator’s Guide

How Long Will an Archive Take?

How Long Will an Archive Take?
Thenumber of variables that you must consider in estimating the time for an
archive make the task more of an art than a science. Each of the following
items has an impact on the time needed to complete the archive:
■Overall speed of the tape device, including operating-system
overhead
■Level of the archive (level-0, level-1, or level-2)
■Volume of used pages managed by OnLine
■Amount and type of database activity during the archive
■Amount and type of database activity in the period since the last
archive
■Alertness of the operator to tape-changing demands
Unfortunately, theUNIX time command cannot help you estimate the time
neededto complete an archive. The best approach is to create an archive and
tryto gauge the time for subsequent archives using the first one as a base of
comparison.
If you create an archive through theDB-Monitor Archive menu, Create
option, messages display the percentage of the archive that is complete.
Thesemessages might help you estimate how much time and tape you need
to complete the archive, but the%done value can be misleading. The
confusionarises because the calculation for %done is based on total allocated
OnLine space, but only used pages are archived.
Considerthis example. A 100-megabyte chunk allocated to OnLine contains
75megabytes of used pages and 25 megabytes of free space, which is located
at the end of the chunk. The archive proceeds at a steady rate as the%done
value climbs from0 %done to 75 %done. Whentbtape reaches the last 25
percent of the chunk, it determines that the remaining pages are free space
and therefore are not archived. The value of%done suddenly jumps from 75
to 100 and the archive is complete.