Subject: Re: Dual SCSI, single chain?
To: NetBSD/alpha Discussion List <>
From: Greg A. Woods <>
List: port-alpha
Date: 03/29/1999 15:31:28
[ On Monday, March 29, 1999 at 07:25:12 (-0500), Curt Sampson wrote: ]
> Subject: Re: Dual SCSI, single chain?
> Really, Greg, none of this is a big surprise. And all of this
> applies even if you have only one device and one controller on the
> bus. It's nothing specific to with mixing multiple devices.

Thanks for re-iterating what I said, but no thanks.

Look Curt, you said at least one thing wrong, and gave a whole lot of
other rather misleading and incomplete advice.  I and several others
just jumped in to correct it and flesh out some details (as well as to
relate war stories!).

> > Even once you got
> > everything right in theory there are some fairly tricky timing issues,
> > not to mention that SCSI-1 and SCSI-2 devices which barely meet their
> > own requirements are unlikely to meet the more stringent requirements
> > necessary for Ultra and Ultra-2 speeds (even if they're off at the
> > "narrow" end of a bus).
> But they won't be using the bus during the sync. phase of the Ultra
> or Ultra-2 devices. That's the whole point of SCSI backwards
> compatability, and why it works.

As Wilko says, there are a multitude of factors that may prevent these
things from working.  I've got at least one Digtial Expansion Chassis
which will not co-exist on the average bus with any FAST-20 devices.

> Anyway, the summary is: give it a go, this stuff can work just fine
> at home. No, it's not going to be the sort of ultra-reliable thing
> you need for production use, but then again, if you're that concerned,
> go buy a DEC Storageworks rather than futzing about with some weird
> conglomeration of drives you found in a dumpster somewhere.

Many non-hardware-knowledgeable people I know just throw out their SCSI
gear because they try to "give it a go" and it doesn't.  However their
problems are usually very minor and with a minimum level of
understanding they're often evident and correctable.  I.e. don't just
tell people to "give it a go" -- give them real information that they
can use.  Gary Field's web pages, that I referred to before, have some
extremely useful rules-of-thumb (The SCSI Game).

							Greg A. Woods

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