Path: utzoo!mnetor!uunet!husc6!mailrus!ames!umd5!purdue!i.cc.purdue.edu!j.cc.purdue.edu!pur-ee!uiucdcs!uiucdcsp!gillies
From: gillies@uiucdcsp.cs.uiuc.edu
Newsgroups: comp.arch
Subject: Re: Disk drives -- speed of?
Message-ID: <76700012@uiucdcsp>
Date: 19 Apr 88 04:04:00 GMT
References: <2746@sundc.UUCP>
Lines: 18
Nf-ID: #R:sundc.UUCP:2746:uiucdcsp:76700012:000:827
Nf-From: uiucdcsp.cs.uiuc.edu!gillies    Apr 18 22:04:00 1988


I want to suggest a possible reason that may be untrue.  A 3600 RPM
disk rotates 60 times per second.  Thus, the maximum access time to
any SECTOR on such a disk is 16.7ms.  The average SECTOR access time
(assuming you start from a random sector) is 8.3ms.

Some of the more sophisticated stepper motors (voice-coil?) in
Winchester disks can do an average TRACK seek in 16.5 ms (CDC Wren).
But why make the disk spin faster, if the head cannot get to the TRACK
quickly?  The average TRACK seek time would have to be 8.3ms before it
would be the main bottleneck in disk random-access performance.

Also, maybe the economics don't make sense.  After all, nearly all the
money in Winchester disks is in the high-volume low-performance
devices.

Don Gillies {ihnp4!uiucdcs!gillies} U of Illinois
            {gillies@p.cs.uiuc.edu}