Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!samsung!sol.ctr.columbia.edu!emory!utkcs2!scion
From: scion@cs.utk.edu (Sam C. Nicholson II)
Newsgroups: comp.periphs.scsi
Subject: Re: WARM SWAP
Message-ID: <1991Apr22.191842.29978@cs.utk.edu>
Date: 22 Apr 91 19:18:42 GMT
References: <22249@shlump.nac.dec.com>
Sender: usenet@cs.utk.edu (USENET News Poster)
Organization: University of Tennessee, Knoxville - CS Department
Lines: 47

In article <22249@shlump.nac.dec.com> guineau@star.enet.dec.com (W. John Guineau) writes:
>
>
>Is anyone using SCSI devices in a WARM SWAP mode? 
>
>COLD SWAP: everything powed off, peripherals swapped, everything powered up.
>WARM SWAP: everything powered on, NO activity on bus, peripherals swapped.
>HOT  SWAP: everything powered on, activity on bus, peripherals swapped
>
>What are the risks associated with this?
>

I was recently writing drivers for WORM drives and would frequently
swap drives around. Can't say as I recommend it as good for the health
of your running system.  The reconnection is going to generate a SCSI
reset.  Some OSs just will not tolerate them.*

I think that I  have treated devices on a SCSI every bit as cavalierly
as some C programmers treat pointers and integers and as hardware
hackers have treated  devices on a UNIBUS.  I have also panic'ed and
re-booted often.  As a powercycle time saver I do feel that I saved
more time than I lost.  Just remember where your terminators are.

It is difficult to say whether there is activity on the bus or not.  I
certainly would not trust the absence of LEDs or the sound of swishing
heads as a certain indicator of a quiecent bus.  If I had an emergency
swap, (e.g. Tape drive failed and I just had to get a crash dump or a
backup without a reboot ) I would halt the processor ( L1-A on my Sun,
^P on my VAX; your mileage may differ) and feel that that would halt
most activity on the bus, do the swap, and continue; hoping for the
best.

I don't beleive that I have read any thing about the SCSI bus that
would give me any confidence in the correctness of dis- and
re-connecting devices with regard to the electrical connections.  I
believe that the standards folk would not assume that activity as normal.

For production use, I would *strongly* recommend removable media
devices.  Sony, LMS, Bernoulli, and Syquest come to mind for disks
Archive, Wangtek and Tandberg likewise for cartridge tapes.

GAWD, I ramble on...
-sam

--------------
* They feel that THEY (being the bus master) have an exclusive domain
over the reset line.  They are wrong, but they are in use.