Path: utzoo!utgpu!watmath!att!dptg!rutgers!tut.cis.ohio-state.edu!gem.mps.ohio-state.edu!csd4.csd.uwm.edu!bbn!oliveb!pyramid!ctnews!mitisft!dold
From: dold@mitisft.Convergent.COM (Clarence Dold)
Newsgroups: comp.periphs
Subject: Re: Hard drive speeds
Message-ID: <1208@mitisft.Convergent.COM>
Date: 31 Aug 89 02:25:36 GMT
References: <6992@sdcsvax.UCSD.Edu>
Organization: Convergent Technologies, San Jose, CA
Lines: 20

in article <6992@sdcsvax.UCSD.Edu>, jc@odin.ucsd.edu (John Cornelius) says:

> In the past 4 years we've seen average seek times drop from around 28ms to
> around 14ms and transfer rates increase from 2Mbps to 3Mbps.  Rotational
> speeds, however, have remained at about 3600RPM giving a rotational
> latency of about 8ms.  As seek times approach rotational latency the speed
> with which the data may be accessed will be dominated by rotational
> latency and controller behavior, not by seek times. 

Here the 'controller behaviour' is very significant.  The SMD (and SCSI?)
controllers from InterPhase even think ahead about latency.
If a request is made for sectors 8-20 on a given track, and the first sector
read is sector 12, the controller will start DMA, with the proper buffer 
offset, eventually finishing the DMA with the transfer of sector 11.
I don't know if this is unique to Interphase, but I thought it was pretty
clever.

-- 
Clarence A Dold - dold@tsmiti.Convergent.COM
		...pyramid!ctnews!tsmiti!dold