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From: dtb@adpplz.UUCP (Tom Beach)
Newsgroups: comp.periphs.scsi,comp.periphs
Subject: Re: What makes a SCSI drive fast?
Summary: minor corrections
Keywords: SCSI drive
Message-ID: <686@adphdw20.UUCP>
Date: 18 Apr 91 22:57:09 GMT
Article-I.D.: adphdw20.686
References: <14971@life.ai.mit.edu> <1991Apr17.232548.24776@colorado.edu>
Followup-To: comp.periphs
Organization: ADP Dealer Services R&D, Portland, OR
Lines: 49

In article <1991Apr17.232548.24776@colorado.edu>, rlr@alumni.colorado.edu (Roger Rose) writes:
> 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?
> 
Stuff deleted
> 
> 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

In MOST cases the servo is not on the same platter as the data. There can
be only one data surface on the same platter as the servo. All other platters
have 2 data surfaces. Hence (platters X 2) - 1 datasurfaces, 1 servo surface.

> due to uneven heat expansion within the drives and wear on the actuator.

Platters and actuators in the middle of the stack get warmer than those
top and bottom. The servo tends to be on the bottom. Track densities today
run > 1600 tracks/in.
> 
> 
> > Must these changes take place at the expense of performance?
> 
> 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.

It's often between ALL sectors. This has a side effect of lowering the 
sustained data rate with the same raw data rate.
 
> 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; therefore, a track switch involves
> relocking the servo.

An interesting point. I've not seen a performance degradation here but
I haven't looked for it either. I think I will... Thanks!

Tom Beach

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|  Tom Beach : Sr Project Engineer : Mass Storage Technology             |
|  phone : (503) 294-1541                                                |
|  email : uunet : dtb@adpplz.uucp                                       |
|  ADP Dealer Services, ADP Plaza, 2525 S.W. 1st Ave, Portland OR, 97201 |
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