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From: fur@ai.mit.edu (Scott Furman)
Newsgroups: comp.periphs.scsi,comp.periphs
Subject: What makes a SCSI drive fast?
Keywords: SCSI drive
Message-ID: <14971@life.ai.mit.edu>
Date: 17 Apr 91 04:00:52 GMT
Sender: news@ai.mit.edu
Followup-To: comp.periphs.scsi
Organization: MIT Artificial Intelligence Laboratory
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I have seen a lot of info posted in this group about SCSI transfer rates. 
However, I have not seen as many postings  about other performance parameters of
SCSI drives.

Recently I was reading some SCSI spec sheets for the Wren IV.  A few parameters
caught my eye:

1) Overhead time for head switch (512 byte sectors):	< 2   ms
2) Overhead time for one-track cyclinder switch    :      6   ms typical
3) Average rotational latency                      : 	  8.3 ms

Two milliseconds to switch heads!?  I would have guessed that switching sense
heads was was done purely by electrical means.  The overhead time for a track
change also seems excessive.  Apparently these overhead times are due to the use
of embedded servo mechanisms which are commonly found now in modern drives.
Embedded servos, I am told, increase reliability and reduce the price per byte. 
Does anyone have an explanation of why embedded servo are more reliable?

Must these changes take place at the expense of performance? Are embedded servos
particularly entrenched in the SCSI world where price/byte is a strong market
force?  Are there any manufacturers who make make drives that don't suffer
these sorts of performance hits?

The 8.3 ms average rotational latency is a result of the standard 3600 RPM.  No
surprises there.  However, I understand Micropolis and HP have introduced drives
that operate at 5400 RPM and 4000 RPM respectively.  Has anyone used these?  Are
any manufacturers coming out with even faster ones?

-Scott