Re: Two hdds on one channel - why so slow?

From: Vojtech Pavlik (
Date: Wed Jan 02 2002 - 13:41:50 EST

On Wed, Jan 02, 2002 at 06:21:25PM +0100, Krzysztof Oledzki wrote:
> On Tue, 1 Jan 2002, Brian wrote:
> > This is an inherent quirk (SCSI folks would say brain damage) in IDE.
> >
> > Only one drive on an IDE chain may be accessed at once and only one
> > request may go to that drive at a time. Therefore, the maximum you could
> > hope for in that test is half speed on each. Throw in the overhead of
> > continuously hopping between them and 12MB is no surprise.
> So?!? This ATA100 and ATA133 standards do not make any sens? It is not
> possible to have more than 66 MB/sec with on drive and is seems that it is
> not possible to use more than ~30MB/sek of 100 or 133 MB/sec ATA100/133
> bus speed with two HDDs. Oh :(((
> Another question - why ATA100/ATA66 HDDs are so slow with UDMA33?
> With new IBM 60 GB IC35L060AVER07-0 I have much more than 33 MB/sec with
> ATA100 and only 24 MB/sec with UDMA33 (Asus P2B with IntelBX). New 80GB Seagates
> (Baracuda IV) have the same problem.

Actually 24 MB/sec is quite a miracle with UDMA33. I'd expect values
around 16 MB/sec. Because, as far as I know, unlike SCSI, IDE doesn't do
concurrent reads and transfers (except for readahead), effectively
halving the interface transfer speed.

Vojtech Pavlik
SuSE Labs
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