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From: brian@casemo.UUCP (Brian Cuthie )
Newsgroups: comp.arch
Subject: Re: Disk rotational speed vs. striping vs. parallel heads
Message-ID: <222@casemo.UUCP>
Date: Mon, 24-Aug-87 09:03:34 EDT
Article-I.D.: casemo.222
Posted: Mon Aug 24 09:03:34 1987
Date-Received: Tue, 25-Aug-87 06:10:24 EDT
References: <2432@ames.arpa> <3721@well.UUCP> <2838@phri.UUCP> <155@dolphy.UUCP> <611@gumby.UUCP>
Organization: CASE Communications, Columbia, MD
Lines: 55

In article <611@gumby.UUCP>, earl@mips.UUCP (Earl Killian) writes:
> In article <219@casemo.UUCP>, brian@casemo.UUCP (Brian Cuthie) writes:
> 
> > More than likely it has six times the drive electronics.  That is it
...	
> I knew you needed multiple head amplifiers, but I didn't think head
> amplifiers were the big cost item in a disk drive (I am a little naive
> on these matters).  Are they really costly?  And six separate

Yes, they are costly.  Also, because of the way data is written to most
drives (MFM) the data must be processed by a seperate SERDES (SERializer
DESerializer) for *each* channel in the controler.  This is a substantial
part of any disk controler and costs $$$ to duplicate.

> > > The problem is that with today's [transfer sizes, parallel head
> > > disks] may not make much difference.
> > This is very untrue in a multi-drive environment.  Most systems have
> > several disk drives under the direction of one disk controller.  This
> > allows the overlapping of seeks so that while a drive is seeking...
> 
> Yes, but the original comment was in the context of speeding up i/o
> for a single application, which I thought to be the motivation behind


I didn't realize we were talking about a single threaded machine.  I guess
it makes less of a difference when one task has to wait for the disks anyway.

> 
> > In all the cases I've seen disk striping was not used to increase mbytes/sec
> > throughput at all.  Rather, striping reduces the number of seeks necessary
> > to access a given chunk of info.  In effect disk striping gives you a disk
> > that looks to have #drives*TRACKS/CYL tracks per cylinder.  This is the
> > *real* advantage to striping.
> 
> I think we're talking about two different mbyte/s here.  You seem to
> be talking about mbyte/s during transfer.  I was talking about
> effective data rates, which includes the seek time.  The whole idea to
> reducing the number of seeks and thus the average seek time is to make
> disk i/o faster, is it not?  I.e. to increase the NET mbyte/s rate
> seen by the application.


Sorry again.  I mis-read ths context of the statement.  I spent several
years designing disk controlers and just assumed we were talking about 
bus bandwidth.  I oftern forget that most people are actually interesed
in throughput. Thus two different Mbytes/s.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Brian Cuthie
CASE Communications
Columbia, Md. 21046

(301) 290 - 7443

UUCP:	...seismo!mimsy!aplcen!casemo!brian