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From: scott@labtam.labtam.oz (Scott Colwell)
Newsgroups: comp.periphs.scsi
Subject: Re: What makes a SCSI drive fast?
Keywords: SCSI drive
Message-ID: <10404@labtam.labtam.oz>
Date: 18 Apr 91 05:31:46 GMT
References: <14971@life.ai.mit.edu>
Organization: Labtam Australia Pty. Ltd., Melbourne, Australia
Lines: 29

In article <14971@life.ai.mit.edu>, fur@ai.mit.edu (Scott Furman) writes:
> 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!?

CDC/Imprimis/Seagate normally quote times _including_ controller
overhead.  Considering that at best it will be an 80188 on the drive
interpreting the SCSI command, these are good figures.  If you weren't
using SCSI the equivalent function must be done on either the controller
card or the host so you don't win either way (unless you have a fast host
with a very dumb controller card.)

The Wren V product spec quotes the head change overhead for a transfer
that crosses onto another surface at 500us. The controller overhead is
usually of the order of ~1ms for the CDC drives.


Scott Colwell

p.s.  Wren drives use a servo track, not embedded servo.