StorageTek Host Software Component 6.0, Руководство системного программиста

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Chapter 6. Performance Considerations 423
1st ed., 6/30/04 - 312579601
Unavoidable Pass-Thrus
The HSC attempts to minimize the number of pass-thrus required; however, depending
upon available tape transports and locations of cartridges, pass-thrus often cannot be
avoided. By running Activities Reports on a regular basis and examining the results, you
can see that mounts for different LSMs take longer than mounts for the same LSM.
Unnecessary Pass-Thrus
Scratch mounts, dismounts, enters, and ejects that require pass-thrus are unnecessary and
should be avoided. These types of activities divert the robot from productive work
especially during periods of peak activity.
If scratch subpools are properly defined and managed, scratch cartridges normally are not
involved in pass-thrus. Scratch mounts are the same as specific mounts as far as pass-thrus
that cannot be avoided. In general, you should never see a scratch pass-thru. But, if you do
see a scratch pass-thru, it was unavoidable.
The effect of unnecessary pass-thrus is not obvious in the Activities Report. You must
compare their number with average mount times to see the effect on performance.
Scheduled Pass-Thrus
Using the Scratch Redistribution utility to balance scratch cartridges involves moving
cartridges to various LSMs to achieve scratch balancing. This type of activity involves
heavy pass-thru usage. If you must balance scratch volumes across your library, schedule
such activity during off peak hours. This approach will ensure that the pass-thru activity
involved does not directly interfere with mounts and dismounts for high-priority
production.
You can use Expert Library Manager (ExLM) to schedule pass-thrus. Refer to the Expert
Library Manager User and Reference Guide for more information.
Ways to Reduce Pass-Thru Activity
There are various ways to reduce pass-thru activity. Each of these ways is briefly
discussed in the following paragraphs.
Set MNTD Float to ON
The Float option of the MNTD command specifies whether the HSC is to select a
new home cell location when it dismounts a volume that requires a pass-thru when it
is dismounted.
When MNTD Float is set to ON, cartridges are not passed back to their original
LSM. The cartridges are assigned new cells in the LSM where they are dismounted.
This action eliminates most unnecessary pass-thrus.
Set MNTD SCRDISM to CURRENT
When MNTD SCRDISM is set to CURRENT, scratch cartridges mounted in a 9360
(WolfCreek) LSM are dismounted in the same device rather than being archived in
the next largest storage device.