Messages in this thread | | | Date | Fri, 20 Jun 1997 14:12:37 -0400 (EDT) | From | Michael Scott Shappe <> | Subject | Re: rescan scsi |
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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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