[3039] in linux-scsi channel archive
Dual-hosting SCSI and failure modes (was Re: RAID & unhappy scsi driver)
daemon@ATHENA.MIT.EDU (Andy Poling)
Mon Jan 5 23:12:32 1998
Date: Mon, 5 Jan 1998 23:08:31 -0500 (EST)
From: Andy Poling <andy@globalauctions.com>
To: linas@linas.org
cc: linux-raid@vger.rutgers.edu, linux-scsi@vger.rutgers.edu
In-Reply-To: <199801060003.SAA21084@shadygrove.linas.org>
On Mon, 5 Jan 1998 linas@linas.org wrote:
> Well, actually ... the reason I want with s/w raid is that I was
> hoping to build a dual-cpu system, with one scsi buss attached to
> two servers. I haven't yet gotten anywhere with this.
>
> While I have your ear, a couple o quickie questions:
>
> -- Having two cpu's accessing the bus at the same time should be OK,
> as long as they are not accessing the same partitions, right?
> That is, the only reason to avoid dual access is to not
> mangle a file system, right?
The drivers and kernel code (mostly) appear to be written under the
assuption that they are the only active host controller on the bus. They do
things like reset the bus when things are going poorly, etc. that might be
considered un-neighborly behavior if there were more than one system and
host controller present...
On the other hand, electronically controlled SCSI bus switches are available
so that the two host controllers need never actually meet on the bus.
If you ask me, a bigger problem for recovery after a failed drive is the
current device naming scheme that could potentially result in *all* of the
remaining disks having different names after one is no longer available
(presently hot swapped out, powered down or otherwise totally unresponsive).
It's this sort of thing that keeps _me_ awake at night...
Related item for the "it would make MD RAID truly awesome" wish list: scan
all present disks for the MD RAID headers (trailers?) and automatically
construct the correct device list to make up each MD device. Other
proprietry logical volume support (SGI XLV) does this, and while it's a
little disconcerting the first time you see it in action, it's very nice to
have when the chips are down.
For now, I can sleep at night because we use a hardware raid controller that
uses a consistent naming scheme of its own for disks.
-Andy
Global Auctions
http://www.globalauctions.com
