[557] in linux-scsi channel archive
Re: Errors & Unit Attention
daemon@ATHENA.MIT.EDU (Michael Weller)
Tue Aug 29 15:26:14 1995
Date: Tue, 29 Aug 1995 10:53:24 +0200 (MSZ)
From: Michael Weller <eowmob@exp-math.uni-essen.de>
To: linux-scsi@vger.rutgers.edu
In-Reply-To: <9508290140.AA25499@pollux.exp-math.uni-essen.de>
> From: sdw@lig.net (Stephen D. Williams)
> Subject: Re: Errors & Unit Attention
> To: bsa@kf8nh.wariat.org (Brandon S. Allbery KF8NH)
> Date: Mon, 28 Aug 1995 10:22:26 -0400 (EDT)
> Cc: sdw@lig.net, eric@aib.com, lnz@dandelion.com, linux-scsi@vger.rutgers.edu
> > acquaintance also fail rather poorly in such a situation. (And for the
> > multi-architecture folks: you can destroy an SBus SCSI card by trying to
> > hot-plug devices...)
>
> Really? Shouldn't be, according to my dim memory of the standard.
> I've also seen it done a lot.
Well hot swapping SCSI is indeed often done, but this is just dancing
with the devil.. it usually works, but you never know. I never saw it as
defined in the standard. (I might have overread it though, but I doubt it).
Instead for whatever scsi-equipment you buy (even for expensive workstation
scsi controllers of about a few thousand $) (and actually esp. for those
as these guys do care more about their customers than vendors of PC cards)
it explicitly warns you to NOT hot swap.
Two mayor problems with hot swap exists: One is protocol related: If some
traffic is on the scsi bus during the hot swap is is certainly interrupted
what may not be detected in all cases, thus data corruption or device
crashes are possible.
The other problems are even worse: Static electricity may discharge
during the hot plug and damage your equipment.
I know not only cases of instantly crashed unix work stations due to a hot
plug but also mysterious dies of scsi-devices (usually adapters, not
external devices) that might easily be related to hot plugging shortly
before the device dead.
From my point of view hot plugging scsi is one of the last tails in this
world. Just don't do it. Even when common OS's (I mean unix
implementations here, not DOS or something similar weird) ALLOW you to
add/delete scsi devices on the fly.
This is not to be confused with raid systems which use special controller
hardware to allow hot plugging (of the raid disks).
Actually one of the advantage of most of these new serial optical busses to
connect storage devices is that these are explicitly hot pluggable.
Michael.
(eowmob@exp-math.uni-essen.de or eowmob@pollux.exp-math.uni-essen.de
Please do not use my vm or de0hrz1a accounts anymore. In case of real
problems reaching me try mat42b@aixrs1.hrz.uni-essen.de instead.)
