Messages in this thread |  | | Date | Fri, 3 Jan 1997 13:10:47 -0500 (EST) | From | "Richard B. Johnson" <> | Subject | Re: Adaptec AHA2940 |
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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.
[SNIP] 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).
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