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From: alexis@ccnysci.UUCP (Alexis Rosen)
Newsgroups: comp.periphs,comp.sys.mac
Subject: Impulse drives from Quantum/Plus
Message-ID: <1397@ccnysci.UUCP>
Date: 12 Mar 89 14:56:33 GMT
Reply-To: alexis@ccnysci.UUCP (Alexis Rosen)
Organization: City College of New York
Lines: 45


A little while ago David Buerger wrote in InfoWorld about these hard disks,
which he claimed would "revolutionize network storage."  These drives are
similar to the regular Quantum 3.5" SCSI drives in aat and capacity (19ms,
40 or 80MB) and the presence of a 64K cache.  Instead of SCSI, they use an
interface called CDI (Cluster Disk Interface).

While these specs are good, I don't understand why these disks impress him.
To start with, he says that the claimed transfer rate is in excess of
4MBytes/sec. This is almost an order of magnitude faster than the transfer
speeds I've witnessed on a Mac II (true, it was using SCSI, but SCSI is
capable of greater speed, even on a Mac II - with the Wrens, for example).

More importantly (and less comprehensibly, to me) is his criticism of SCSI.
   "The CDI data transfer rate is 512 bytes
    per transaction vs. SCSI's 1 byte."
Is that true? It sounds utterly bogus to me (who in their right minds would
create a protocal with 1-byte transfers?) but I'm not a SCSI expert.

Buerger also says that distributing tasks over multiple spindles helps
performance. This is ancient wisdom, to say the least. The problem is that
he says that this strategy is NOT useful over SCSI because there will be
contention between the drives:
   "[SCSI's] weakness is that SCSI is a device interface.
    This results in slower data transfer due to
    translation overhead as multiple drives contend
    with each other."
This appears to be several kinds of nonsense.  What is "translation
overhead?"  Does he mean collision?  I don't think that can happen on
SCSI, especially if there's just one master device (tha Mac, or an IBM
AT controller card).

Now I know that when you have separate controllers on, say, a VAX, you
can improve performance, but ultimately the bottleneck of one bus is
going to hit you. And this impulse drive uses only one controller, also.
So it's limited in exactly the way SCSI is.  (Which is not to say that
multiple spindles won't still be a big win.)

So what's going on? Is he talking through his hat, or am I totally wrong?
I know enough about this stuff to get myself into trouble, but not quite
enough to get out...

Thanks
Alexis Rosen
alexis@ccnysci.uucp