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Date:      Sun, 7 Sep 1997 17:11:10 +0930
From:      Greg Lehey <grog@lemis.com>
To:        Simon Shapiro <Shimon@i-connect.net>
Cc:        FreeBSD Chat <chat@FreeBSD.ORG>
Subject:   Re: lousy disk perf. under cpu load (was IDE vs SCSI)
Message-ID:  <19970907171110.27847@lemis.com>
In-Reply-To: <XFMail.970906235822.Shimon@i-Connect.Net>; from Simon Shapiro on Sat, Sep 06, 1997 at 11:58:22PM -0700
References:  <199709070512.AAA00465@dyson.iquest.net> <XFMail.970906235822.Shimon@i-Connect.Net>

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(moved to -chat)
On Sat, Sep 06, 1997 at 11:58:22PM -0700, Simon Shapiro wrote:
>
> Hi "John S. Dyson";  On 07-Sep-97 you wrote:
>
> What you are all running into is the fact that, for a smoothly run system,
> you need a certain balance between I/O, memory (capacity & bandwidth,
> both), and CPU.
>
> If you compare a 14" disk pack from the mid-seventies (SMDE) to a disk
> drive of today, you see that capacity climbed nicely (about 30x),
> performance has barely moved (I am NOT talking about 5MB Shugart 5.25"!),
> being that a good SMDE drive could do about 5MB/sec or even better, while
> CPU's jumped almost 300x!

Well, I can't remember the performance of the mid-70s, but my
recollection of the performance in the early 80s on, say, a 3330 clone
was that these drives had 30 sectors (2 spares) per track, and they
ran at 3600 rpm.  Since they weren't buffered, that gives a maximum
data transfer to the channel of about 860 kB/s.  Average positioning
was round the 30 to 35 ms mark.

At the time, I was working for Tandem.  We noticed a puzzling
behaviour: our new flagship model TXP, whose CPU was about 100% faster
than the previous NonStop II, read data off these disks at almost
exactly double the speed of the NonStop II.  Why?  We finally figured
out that we were reading 4 kB blocks (the maximum our disk controllers
would allow), doing some processing, and then issuing the next read.
Unfortunately, on the NonStop II this took such a long time that the
head had passed the next block before it issued the read.  In other
words, the "high performance" TXP managed to read a 4 kB block every
revolution, and the NonStop II needed two revolutions per 4 kB block.

This may sound funny until you look at the data transfer rates
involved.  On the TXP, it was 240 kB per second, on the NonStop II it
was 120.  Raw disk rate.

> BTW, the architecture (in realizable terms) of the typical disk controller
> has not changed at all since the IBM-PC, and even the best caching
> SCSI controller is primitive when compared to a 1963 IBM mainframe, and not
> much faster.

Again, I can't agree in the slightest.  I *do* have the technical doc
for a 2311 drive floating around somewhere in the shed, and they were
unbelievably primitive. 

> The young ones amoung you, I have a challenge for you (old folks like me
> can think no more :-):

You're not the only old fart around here.

> Don't bellyache about disk performance.  It will just get worst
> (at 50%/year, last I checked).
>
> Come up with a new I/O ARCHITECTURE.  Totally new.  The first one to come
> up with a truely random storage device with capacity of 1GB and performance
> of (sustained) 100MB/Sec

Sounds like a 1 GB RAM to me.  Still cheaper per byte than any disk
made up to about 5 years ago.

> will make more money than BG thought exists.

Not if BG has anything to say in it.

Greg



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