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Date:      Wed, 25 Feb 1998 19:33:12 +0100 (MET)
From:      Wilko Bulte <wilko@yedi.iaf.nl>
To:        scsi@FreeBSD.ORG
Cc:        tom@sdf.com, blkirk@float.eli.net, hackers@FreeBSD.ORG, scsi@FreeBSD.ORG
Subject:   Re: SCSI Bus redundancy...
Message-ID:  <199802251833.TAA01362@yedi.iaf.nl>
In-Reply-To: <199802251356.FAA18645@silvia.HIP.Berkeley.EDU> from Satoshi Asami at "Feb 25, 98 05:56:01 am"

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As Satoshi Asami wrote...
> (Redirected to -scsi)
> 
>  *   SCSI adapter rarely fail.
> 
> By the way, we're more concerned about network switch/cable/PC/power
> supply failures, rather than adapters.  There are so many things that

Use a hot-swap disk chassis with redundant power supplies. Hook those up
to a *good* UPS. Regularly check you lead-acid creeps (the UPS batteries).

> can take down a machine, and if it's the single connection from the
> disk to the network, it's not good.

Multiple adapters on a single disk do not buy you much either. If adapter
A decides to commit suicide and locks up the bus real good adapter B is
still stuck.

You want some kind of a bus isolator thingy. The company I work for (DEC)
is releasing what is called SCSI hubs. These are multiport devices that
can 'star couple' (for lack of a better terminology) multiple scsi buses,
with each bus being electrically seperated. The larger hubs have 
a round robin arbitration builtin that makes sure that the host with ID 7
will not monopolise the bus to the device.

All of this is based on a chip jointly developed by Digital and Symbios
See www.symbios.com for more info.

Wilko
_     ______________________________________________________________________
 |   / o / /  _  Bulte email: wilko @ yedi.iaf.nl http://www.tcja.nl/~wilko
 |/|/ / / /( (_) Arnhem, The Netherlands - Do, or do not. There is no 'try'
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