Subject: Re: Dual SCSI, single chain?
To: NetBSD/alpha Discussion List <>
From: Curt Sampson <>
List: port-alpha
Date: 03/28/1999 19:31:23
On Sun, 28 Mar 1999, Greg A. Woods wrote:

> Also, unless you're using full differential, or LVD, there are many
> restrictions on the mixing of "slow", "fast", and "ultra-fast" devices
> on the same bus...

No, this is certainly not the case. You can mix any combination of
stuff you want (excepting differential)--narrow, wide, slow, fast,
ultra, etc., so long as you don't mind the bus contention (i.e.,
a 4 MHz CD-ROM drive transferring 1 MB/sec is going to use a quarter
of the bus bandwidth regardless of the width and speed of the
controller and other devices on the bus). In fact, LVD is the only
one you can't mix without losing capabilities (LVD devices revert
to single-ended if there are any non-LVD devices on the bus), and
differential is the only one you can't mix at all.

Given that, yes, you probably do want to segregate your drives on
to different buses to avoid performance limitations, if you've got
low-performance stuff on the bus that you'll actually be using. (I
keep an old 2X CD-ROM on the same bus as my pair of fast new
Barracudas, but I almost never use it, so it doesn't bother me.)

> ...and given that most "narrow" devices are always either
> "slow" or at best "fast"...

Again, no. Virtually all but very old narrow disk drives are fast
SCSI (10 MHz). A lot of CD-ROM drives appear to be slower, though
(4 MHz).

Curt Sampson  <>   604 801 5335   De gustibus, aut bene aut nihil.
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