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From: taylor@intellistor.com (Dick Taylor)
Newsgroups: comp.periphs.scsi
Subject: Re: What makes a SCSI drive fast?
Keywords: SCSI drive
Message-ID: <1991Apr19.164344.3782@intellistor.com>
Date: 19 Apr 91 16:43:44 GMT
References: <14971@life.ai.mit.edu>
Organization: Intellistor, Longmont, CO
Lines: 45

In article <14971@life.ai.mit.edu> fur@ai.mit.edu (Scott Furman) writes:
>
>Recently I was reading some SCSI spec sheets for the Wren IV.  A few parameters


Remember, the Wren IV is an OLD drive (designed before CDC MPI became
Imprimis which became Seagate).  I remember first seeing one some
four or five years ago, which makes it antique in the 5.25" world.

>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. 
Actually, a lot of companies don't use embedded servos because they're
slower and the reliability advantages are debatable.  (And they
make reformatting at different sector sizes impossible, which is a real
competitive disadvantage).

>...
>force?  Are there any manufacturers who make make drives that don't suffer
>these sorts of performance hits?

Lots of them.  Including Seagate.  You need to find some more modern drives
to look at.

>
>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?
>

I don't know personally of any rotating at over 5400.  Greater-than-3600
is becoming pretty common.

>-Scott

			-- Dick Taylor