From: sdesmara@sobeco.com (s.desmarais)
Subject: Re: Powering SCSI devices
Organization: Sobeco Ernst & Young
Date: Thu, 3 Sep 92 16:59:09 GMT

In <1992Aug25.132239.2900@awdprime.austin.ibm.com> shieh@austin.ibm.com writes:

>In article <2A98ED54.64FC@tct.com>, jim@tct.com (Jim Kunzman) writes:
>> A while ago I posted an article about removing devices from a live
>> SCSI bus.  The general concensus was that this is not a good idea and
>> I concur completely, since random noise might cause a corrupt write to
>> a disk drive.
>> 
>> Over the weekend I read an article in Byte about the "greening" of
>> computers, an article about the environmental concerns that
>> microcomputers have surfaced.  One topic was power consumption.  I
>> immediately thought about the problems we have with SCSI devices and
>> wondered if it would be possible to shutdown SCSI devices when they
>> aren't being used, e.g. scanners, printers, tape backup units, MO
>> drives, etc.
>> 
>> Jim Kunzman at Teltronics/TCT     <jim@tct.uucp>, <uunet!pdn!tct!jim>

>I belive, if I remember my SCSI rules, that powering off the
>SCSI device shouldn't matter in terms of the termination.  Part
>of SCSI says that you *will* power the target devices for termination
>even though they may be switched off.  So juice is still going through
>the cable to your device just for termination's sake.  But the
>act of powering off might cause a line spike, which in turn might
>lead to a Bus Reset, depending on your hardware/software implimentation.
>-- 
>The views expressed are my own.  They do not represent IBM.
>Johnny Shieh (shieh@racerx)	Worker Bee, AIX Field Quality (level 3)
>VNET:     SHIEH at AUSVMQ	Internet: shieh@aixwiz.austin.ibm.com

This last pararaph by Johnny Shieh explains exactly what we're experiencing
here on our MIPS 3240 and 3330.  We frequently get our exabyte tape
drives in a bad state: they won't unload the tape by themselves using
the offline command, or they'll just plain have an "hardware/parity/... error"
(or some error message similar to that).

Before, we were powering down the whole computer, which is a real nuisance.
Now, we switch the device off for 10+ seconds, then back on.  Most of the
time, it works, but sometimes, there's a Bus Reset, which is not that bad
for the disks: the controller is bright enough to reissue the command if
needed, but has a more dramatic effect on other tape drives: they instantly
stop and rewind any tape they had, thus causing the backups being done on
them to abort (we have a tape server with 3 exabytes).

Somebody else spoke about if the drive you're powering down provides 
terminator power, this would change the electric characteristics of the
bus.  I would believe so, unless the device continues to provide TERMPWR
even when switched out (which is a bit difficult if you switch off the
power supply itself :-)

In another article, Guy.Kendall@ColoSpgs.NCR.COM wrote:

> It really depends on the SCSI controller chip. In all our newer chips
> we put in additional power-up/power-down circuitry that would gaurantee
> that our chips will not glitch the SCSI bus when being powered up or
> powered down.

Do you make exabytes ? :-)
-- 
Stephane M. Desmarais               sdesmara@sobeco.com  ou  sdesmara@sobeco.ca
Division STS - Sobeco Ernst & Young {uunet | mcgill-vision}!sobeco.com!sdesmara
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