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From: rlr@alumni.colorado.edu (Roger Rose)
Newsgroups: comp.periphs.scsi,comp.periphs
Subject: Re: What makes a SCSI drive fast?
Keywords: SCSI drive
Message-ID: <1991Apr17.232548.24776@colorado.edu>
Date: 17 Apr 91 23:25:48 GMT
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In article <14971@life.ai.mit.edu> fur@ai.mit.edu (Scott Furman) writes:
> Does anyone have an explanation of why embedded servo are more reliable?

Better worded that embedded servo "may" be more reliable.  The answer
here is mainly because the servo information is closer to the data.
The real answers ought to come from the servo gods, but I'll do my
best here.

Non-embedded servo dedicates a separate head to servo.  The servo
pattern is separated from the actual data.  Quite possibly, the servo
isn't even on the same platter as the data.  This runs into problems
due to uneven heat expansion within the drives and wear on the actuator.

Embedded servo mixes servo and data onto the track.  This way, you can
position with the same head that is reading the data.  This opens up
another reliability problem, the head has write current connected and
can accidently destroy the servo information.  This is more severe than
nuking sector headers.  On extremely high-density drives, loss of servo
means returning the HDA to the factory or replacing it, since the
standard mechanics cannot position accurately enough to write servo.
(Of course, there are interlocks to prevent overwriting servo, but
active components aren't nearly as reliable as simply grounding the
wire during assembly.)

> Must these changes take place at the expense of performance?

I don't know about "must", but I've not personally worked with a system
where it didn't.  One obvious hit with embedded servo is it isn't always
there when you want it.  You have to wait for it to come under the head,
just the same as you wait on a sector.  Servo is generally replicated
around the track to minimize delay, but it's still slower than dedicating
entire tracks to servo data.

Another issue involves how often you need to look for servo.  With
dedicated servo tracks, you only need to look for servo when you switch
to a new servo track.  (Typically, this is on cylinder switches, but
some drives have multiple servo tracks per cylinder to improve positioning.)
Embedded servo may appear on every track; therefor, a track switch involves
relocking the servo.

-- 

Roger Rose  {rlr@boulder.colorado.edu}