On Tue, Jan 16, 2001 at 01:22:59PM -0500, Thor Lancelot Simon wrote:
> This is a poor idea, because of the way UDMA IDE controllers work.  They
> aren't really intelligent at all -- you ask them for blocks, and they
> DMA them to you, then whack you with an interrupt when they're done.  They
> don't have any buffering, and if a transfer can't get to you quick enough
> because the PCI bus is in use by another device, everything stalls -- they
> may even have to reissue commands to the disk, though I can't confirm that
> this ever happens.
> 
> With disks that can go 30-40MB/sec, a maximum request size of 64K, and a 
> maximum real-world PCI bus throughput in the 80-100MB/sec range due to
> limited burst length, you'll see a *lot* of PCI bus contention, meaning
> that those IDE controllers are all going to spend a *lot* of time idle.
> 
> In my experience, even with one disk per channel, I never do better than
> about 50-60MB/sec with two disks; with four disks, throughput is *worse*
> than with two, though obviously there are twice as many heads available
> so random I/O does somewhat better.

It's a poor idea to put 2 disks on one channel because of performances
and possibly stability problems. But with one disk per channel,
5 Ultra/66 disk I can push my celeron500 (i810-based motherboard) to
100MB/s.

--
Manuel Bouyer <bouyer@antioche.eu.org>
--