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From: williamt@athena1.Sun.COM (William A. Turnbow)
Newsgroups: comp.unix.xenix,comp.unix.i386
Subject: Re: ESDI vs SCSI
Message-ID: <98520@sun.Eng.Sun.COM>
Date: 12 Apr 89 00:59:44 GMT
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Reply-To: williamt@sun.UUCP (William A. Turnbow)
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Organization: Sun Microsystems, Mountain View
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	Don't know if anyone had followed this up, but my experience with
ESDI/SCSI drives leads me to believe that ESDI drives are faster.

	First off you have limiting factors in bus transfer speed.  On an
AT bus I believe this is something like around 4-5 Meg/sec.  Neither of
these drives comes close.  The limiting factor from what I have seen is
interleave, # sectors/track and (rarely) spin rate.  The spin rate is
almost always 3600 RPM or 60 RPS.  Using this one can quickly see the
maximum sustained xfer rate for a drive with 17 or 34 sectors per track
(512 bytes/sec * # secs/trac * 60 RPS)  = 522K/sec max for standard MFM
and 1044K/sec max for standard ESDI.  I haven't seen any standards on how
many secs/track the SCSI drives have (in the small form factors), but
the transfer rates were about 3/4 to '=' the ESDI's we evaluated.  Also, SCSI
controllers have *in the past* typically had about a 2-3 millisecond
processing delay added on to commands because of the drives greater
intelligence.

	The synchronous transfer rate for SCSI is meaningless if it can't
get it off the disk.  Of course if the SCSI controller has an on
controller cache of some reasonable size, and perhaps a track read
ahead then you might get better performance...

-wat-