Re: Adaptec AHA2940

Richard B. Johnson (
Fri, 3 Jan 1997 13:10:47 -0500 (EST)

On Fri, 3 Jan 1997, Andrew E. Mileski wrote:

> > > I've been using an AHA2940 for ages, and it works fine. However, there are
> > > some problems with changing SCSI adapters, e.g. a harddisk formatted with
> > > an Adaptec controller sometimes won't work with a NCR controller, and vice
> > > versa.
> >
> > b..b..b..b..But... SCSI is supposed to be interchangeable, a fundamental
> > goal of the interface design. This is alarming news. Is NCR or Adaptec
> > at fault?
> Neither - this is universal in my experience. Moving from one model
> of controller to another OF THE SAME MAKE can suffer from the same
> problem. Can't say I know why though.

The problem is with the controller. Confirmed. But it's not a problem
with formatting. It's a problem with building a file-system. So I don't
blame Linux drivers for anything, lets say we are talking DOS and we
use the BIOS provided with the controllers.

Most expensive controller/BIOS combinations do scatter/gather to queue
reads and writes to minimize head movement. Let's say that I don't really
know the geometry of the drive so I make a guess that there are really
41 sectors to a physical track, rather than 64, 128, 256, etc., that
are used for logical sectors. Now, in the process of deferring reads
and writes, I have to do some integer math to define which ones I read
or write first. I make a scatter/gather table and, as long as each sector
is unique, disk access will work. Note that in the example "41" is not
a factor for 64, 128, or 256, etc.

Now however, If another controller/BIOS is used, that has a different
scatter/gather translation, data is not where you think it should be.

Therefore, when using scatter/gather, you have to access a file system
using the same controller upon which it was built.

This is not a SCSI problem or a Disk problem. It's a BIOS problem (or
a driver problem).

Dick Johnson
Richard B. Johnson
Project Engineer
Analogic Corporation
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