IBM 000-8697 User Manual

IBM Informix OnLine Database Server Administrator’s Guide

The number 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, the

UNIX

 time command cannot help you estimate the time

needed to complete an archive. The best approach is to create an archive and
try to gauge the time for subsequent archives using the first one as a base of
comparison.

If you create an archive through the

DB-M

onitor Archive menu, Create

option, messages display the percentage of the archive that is complete.
These messages might help you estimate how much time and tape you need
to complete the archive, but the

%done

 value can be misleading. The

confusion arises because the calculation for

%done

is based on total allocated

OnLine space, but only used pages are archived.

Consider this example. A 100-megabyte chunk allocated to OnLine contains
75 megabytes 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 from

0 %done

 to

75 %done

. When tbtape 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.