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[OM] Re: OT a couple computer questions

Subject: [OM] Re: OT a couple computer questions
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxx>
Date: Fri, 18 Aug 2006 07:41:31 -0400

Bart Wientjes wrote:

> (E)IDE and ATA were designed and manufactured in order to connect two disks
> to one channel, using one cable. Generally this was regarded as a
> non-desirable configuration, as the standard does not allow for the two
> disks to be addressed simultaneously.

This part I agree with.  No parallelism on the same cable.

> Usually one controller has two channels. I think attaching the two disks to
> separate channels of the same controller does allow for paralellism.
> The situation for sata is practically identical, but they skipped the
> two-drives-on-one-channel bit that was a bad option anyhow. And they made
> the cabling more flexible :)

This is the part I was unsure of.  I don't think (say 15 years ago) that 
there was any parallelism here either.  I think the controller could 
only handle one drive at a time using cable select and then drive select 
on the cable.  But with two cables there was the opportunity for 
parallelism by enhancing the controller.  What I was asking (which 
assumed I was correct about how it used to be) was whether current ATA 
and SATA controllers do support parallelism.

> 
> So if you want to have many disks, you need to have many controllers.
> SCSI of course still is an option. With SCSI you can drive multiple devices
> simultaneously in the same channel (yeah I know it's called a bus), its just
> that you are sharing bandwith.
>

SCSI trivia.  SCSI design was based on the early IBM 360 I/O channel design.

Chuck Norcutt


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