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From: jlohmeye@entec.Wichita.NCR.COM (John Lohmeyer)
Newsgroups: comp.periphs,comp.sys.mac
Subject: Re: Impulse drives from Quantum/Plus
Message-ID: <324@entec.Wichita.NCR.COM>
Date: 13 Mar 89 22:44:06 GMT
References: <1397@ccnysci.UUCP>
Reply-To: jlohmeye@entec.UUCP (John Lohmeyer)
Organization: NCR Corporation, Wichita, Kansas
Lines: 59

In article <1397@ccnysci.UUCP> alexis@ccnysci.UUCP (Alexis Rosen) writes:

>A little while ago David Buerger wrote in InfoWorld about [..stuff deleted..]
>an interface called CDI (Cluster Disk Interface).
  [more stuff deleted]
>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.

I am a SCSI expert and it is indeed bogus. SCSI places no restrictions over
how much data can be transferred at one time (although there is a method for
a host to limit the transfer size for performance tuning).

>Buerger also says that distributing tasks over multiple spindles [...]
>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).

SCSI certainly can support multiple spindles per bus and many SCSI devices
also support multiple spindles per target (SCSI buzz word for a controller).
SCSI is NOT a device level interface.  Most SCSI implementations are buffered
so that bus contention is not as serious a problem as on a device level
interface where contention means slipped latencies.

And yes, contention can occur with only one host.  If the host supports
multi-tasking several controllers can attempt to reconnect (another SCSI buzz
word for trying to obtain the bus to continue an I/O) at the same time.
Buffered controllers take a lot of the sting out of contention.


 [more stuff deleted]
>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

My best guess is that David Buerger doesn't really understand SCSI.  I did
not read his original article, so I don't know much about the CDI he was
promoting.  Perhaps CDI has some nice features, but by incorrectly
criticizing SCSI he has only convinced me that he doesn't know what he
is talking about...  I would like to see some specs on CDI to make
my own decision.

___________________________
John Lohmeyer
NCR Corporation
J.Lohmeyer@Wichita.NCR.COM