The Scsi Bus And This Jukebox; Mixing Wide And Narrow Devices - HP SureStore 220mx User Manual

The SCSI Bus and This Jukebox

This jukebox is a "fast and narrow" SCSI device that uses either a
single-ended or low-voltage differential external SCSI bus. The external
connectors on the jukebox are "wide" (68-pin) but, because the internal
SCSI bus in the jukebox is "narrow," only eight data lines are available
for addressing. The jukebox may only use SCSI IDs from 0-8 (not 0-15
available on "wide" buses). The jukebox does not support logical unit
number (LUN) addressing.
When calculating available cable length, subtract 16 inches (0.41 metre),
the length of the SCSI bus used internally by the jukebox.
When considering adding other peripherals to the jukebox bus, you
should consider the following:
• Will adding a device onto a bus with this jukebox interfere or degrade
the performance of the jukebox or the device that is added?
• If the decision is made to add other devices to the jukebox bus, what
SCSI requirements must be observed to make sure the bus will work?
Before addressing some questions involved in deciding whether to place
another device on the jukebox bus, we can say that, as a general rule,
HP recommends that the jukebox be the only device on a bus. If there are
multiple devices on the bus, performance of one or all of the devices on
the bus can be affected.
If you are still considering adding a device, you might first consider the
quantity of data that the bus will have to carry, the frequency of data
transfer, and the priority of this data transfer in your business. If you are
using the jukebox as a backup or archive device, the demands on the
jukebox will be different than if the jukebox is used for near-online
storage where requests are more random in both frequency and data
size. If you expect to be using the jukebox in the same time-frame as the
other device, or devices, this might indicate that you may experience a
drop in performance.

Mixing Wide and Narrow Devices

As a preliminary note to this discussion, HP highly recommends that you
do NOT mix wide and narrow buses.
Appendix B
Operating This Jukebox on a SCSI Bus
The SCSI Bus and This Jukebox
B-7