I've dealt with this exact issue many times before. It?s a balancing act that
really boils down to this:
Only one restore job can access the same tape at a time, and each restore job
can only use one tape at a time (there is a new feature for doing parallel
restores that were multiplexed, but that only applies to certain situations
that databases don?t fall under). The best solution for this is to duplicate
all the images on the tapes to a DSU (disk storage unit), then all the restore
jobs can access the DSU all at the same time. Make sure you duplicate the
entire tape (not individual images) to get the duplication done as quickly as
possible. This is also one of the reasons why having your on-site backup copy
be on disk is getting to be so popular. In fact, I wouldn?t even consider
doing hourly database log backups directly to tape.
Also, the less often you do your log backups, and the more logs you get backed
up in a single stream (meaning fewer backup images) the faster your restores
will be.
To answer your questions:
1-2. Volume Pools can help you divide up your data and improve performance, if
done correctly, but it can also cause you to use more media with tapes not
getting filled up. Again, it?s a balancing act, but most place I?ve worked at
are more concerned about costs than performance (at least until they need
something restored ASAP).
3 & 5. You can only specify storage units for backups (not specific tape
drives), so you have to configure your storage units accordingly if you want to
restrict which drives get used for what. On the other hand, restores only care
which server they were backed up on (I'm not familiar with a way to force the
use of a certain storage unit for a restore). You may want to make a close
inventory of your storage units to see why you always have a couple of drives
available; it may have been designed that way on purpose to allow for restores
or user initiated backups to be done at any time.
4. Especially with Databases, it's a delicate balancing act. Improving one
often has a negative impact on the other; again using disk staging areas or VTL
can help a great deal with this by eliminating the need for multiplexing and
improving overall performance. Just remember tapes are linear, so to get to
the data at the end of the tape, means the drive has to pull the entire tape
through it, and it can only read one part of the tape at a time.
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