Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!tut.cis.ohio-state.edu!purdue!bu-cs!encore!pinocchio
From: terryk@pinocchio (Terence Kelleher)
Newsgroups: comp.periphs
Subject: Re: SCSI->SCSI vs. SCSI->ESDI disk performance?
Message-ID: <10959@xenna.Encore.COM>
Date: 20 Jul 89 15:45:30 GMT
References: <398@sunny.ucdavis.edu> <1989Jul19.170414.20326@utzoo.uucp>
Sender: news@Encore.COM
Reply-To: terryk@pinocchio (Terence Kelleher)
Organization: Encore Computer Corp
Lines: 43

In article <1989Jul19.170414.20326@utzoo.uucp>, henry@utzoo (Henry Spencer) writes:
>
>The odds are very good that your "SCSI disk" is in fact an ESDI disk or
>something similar with such a translator built in.  No disk drive directly
>talks SCSI; there has to be a controller in there somewhere, even if it's
>built into the drive.  Many manufacturers supply the same drive as a SCSI
>drive or an ESDI (or whatever) drive simply by leaving out the controller
>board and the SCSI connector if the order doesn't say "SCSI".

I'll probably get in trouble for this, but not all SCSI drives
translate to another standard.  The Wren series from
CDC/Imprimis/Seagate (whatever they are this week) is clearly a direct
SCSI disk.  The SCSI drives are formatted using Zone Bit Recording,
which modifies the access clock in Zone areas to increase the number
of sectors per track on outer cylinders, increasing both capacity and
average transfer rate. This is not possible with ESDI, SMD or any
other disk specific interface.  The SCSI drives have only one
controller card. The SCSI and ESDI versions of a given Wren product do
not nessecarily have even the same HDA, much less share electronics.

>
>*If* the ESDI drive is hooked directly to your machine, not via SCSI.
>Running it through a SCSI translator eliminates any inherent performance
>gain.
>
>(Actually there is no inherent reason why SCSI drives can't be blazing
>fast, it's just that most of them aren't.)

If you have multiple disks on your system, the SCSI interface disks
may well outperform the ESDI interface disks.  The SCSI bus can
transfer at a faster rate than the ESDI disks, and since the SCSI
controllers typically buffer data, the transfer on the bus does not
need to take place in sync with transfer from the heads.  Multiple
disks can overlap operations and yeild a higher agragate throughput.

>-- 
>$10 million equals 18 PM       |     Henry Spencer at U of Toronto Zoology
>(Pentagon-Minutes). -Tom Neff  | uunet!attcan!utzoo!henry henry@zoo.toronto.edu
-- 
Terry Kelleher, Encore Computer
Phone: 508-460-0500
UUCP: {bu-cs,decvax,necntc,talcott}!encore!terryk
Internet: terryk%pinocchio@multimax.ARPA