[3048] in linux-scsi channel archive
Re: RAID & unhappy scsi driver
daemon@ATHENA.MIT.EDU (linas@linas.org)
Tue Jan 6 17:28:26 1998
To: dledford@dialnet.net (Doug Ledford)
Date: Tue, 6 Jan 1998 16:50:28 -0600 (CST)
Cc: linas@linas.org, linas@fc.net, linux-eata@trudi.zdv.Uni-Mainz.DE,
linux-scsi@vger.rutgers.edu, linux-raid@vger.rutgers.edu
In-Reply-To: <XFMail.980106005146.dledford@dialnet.net> from "Doug Ledford" at Jan 6, 98 00:13:26 am
From: linas@linas.org
It's been rumoured that Doug Ledford said:
>
> More or less, the aic7xxx driver should fairly well handle reservation
> conflicts and busy devices.
Good!.
> >-- the bus-busy signal wire can be cleared with a bus reset, right?
> > I figure that if the main server goes down, its power will be
> > cut with some dead-man switch, and the other CPU can take over.
> > I was not anticipating power-cycling the disk enclosure.
>
> Unfortunately, no. It isn't that easy. In this case, the Busy pin on the
> SCSI bus is a shared signal pin. Its active state is defined as active
> *LOW* meaning 0V nominal, but actually, anything under about 2V should
> trigger devices into thinking the bus is busy). Any device is allowed to
> pull this pin low (active) in order to signal busy. The power to drive this
> pin high (inactive) is provided by the SCSI terminators. For certain
> failure modes, power cycling won't even help. For example, if one of your
> drives develops a physical short between the busy signal pin and ground,
> that pin is going low and it wont go high again until you disconnect that
> device from the bus, period, end of discussion, system dead. You could
> power cycle things until you're blue in the face and it wouldn't help. So,
> knowing this information, you can see, a single SCSI bus is *NOT* adviseable
> for a truly failsafe operation.
Hmm,
I guess something like a 10baseT hub with fault isolation for scsi
would be neeeded. But I've never heard of such a thing.
Alternately, I imagine that maybe IBM SSA architecture disks don't suffer
this problem, given that they are meant for HA deployment. I though
I heard of someone doing an SSA driver for linux, but suspect its not done.
Also, my impression is that ssa disks are hard to find and very expensive.
--linas
