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SubjectRe: rescan scsi
On Fri, 20 Jun 1997, docwhat@gerf.org ;The Doctor What wrote:

> I have a question....why do people say NOT to add scsi devices on the
> fly to a system? I thought that was (inpart) the point of SCSI?

I've never heard anything to suggest that part of the point of SCSI was
hot-swap capability. Most of the SCSI specs pre-date a strong demand for
hot-swapability -- or perhaps they merely predate the PERCEPTION of such a
demand, since it seems to me that Typical User Cluelessness combined with
Murphy's and Finagle's Laws, guarantees that if you say, "don't do that",
many people will do it. Arguably, hot-swapability should be a standard
feature of all peripheral interconnects, if for no other reason than to
protect users from themselves :-)

(Perhaps you're having acronym confusion? Part of the point of PCMCIA is
hot-swap, for example...)

Certainly, there are lots of nice devices out there now that allow you to
do some kind of hot-swap with SCSI devices, and some vendors (like Sun)
use a special (non-standard...grrrr) connector to make internal and array
drives hot-swapable and, incidentally, also easier to install.
As someone else mentioned, there's a small amount of danger in
hot-swapping *any* connection that isn't expliclty rated as hot-swapable.
Example: on most Apple hardware for the last decade, keyboard and mouse
(and other input devices) all hang off an "Apple Desktop Bus".
Hot-swapping devices on this relatively simple bus can toast the ADB
controller, rendering your machine "deaf" to keyboard and mouse events. Of
course, six times out of ten, this probably won't happen, and everything
will just work; and three times out of ten, your OS will hang, but no
hardware will be roasted; but you'll really hate yourself if you happen to
hit that one-in-ten that lets the Magic Smoke out, now, won't you :-)

This is also true of simple serial connections, parallel connections,
anything. If it doesn't say "hot swappable", it isn't, and while the
danger may be small (I don't know if I've ever actually heard of anyone
setting their machine on fire by hot-adding a modem to an RS-232 port,
f'rinstance), it's there, nonetheless.

--
Michael Scott Shappe <mshapp19@idt.net>
Web: <http://www.publiccom.com/web/mikey/>
Addicting the unsuspecting to the net since 1987


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