[geeks] q about connecting external SCSI array

[der Mouse]

> Can I connect the external [SCSI] array [to a lvie system] without
> having to reboot or will something bad happen should I try to do
> that?

"Maybe."

With traditional SCSI, SCSI which is electrically a single bus, this
runs the risk of corrupting any operation that is in progress when
you're making the connection.  Quiesce the bus (eg, break to ddb in
NetBSD) and you likely can plug it in then.

However, there may well be termination issues.  In some cases you can
deal with this yourself (remove a terminator and plug in a cable
terminated at the other end).  Some devices auto-terminate in hardware,
in which case it should Just Work.  If the device self-terminates but
depends on software to do it, you may have problems.

And some SCSI interfaces are _not_ electrically a single bus; I've seen
some that appear to the host as a single bus but I believe are
electrically three different buses, with jiggery-pokery in the driving
electronics to generate that illusion of a single bus.  This is a
little like how Ethernet switches make multiple physical segments look
like a single segment in some respects - but, for example, collision
domains do not match broadcast domains the way they do with real
10base5.

Since the OS supports live rescanning, it's reasonably likely the
drivers do whatever is necessary to support it on that hardware.  You
may be able to get away with just plugging it in, but I'd try to find a
way to quiesce the bus first.

I've also seen it said you can blow terminator power fuses this way.
I've never had it happen, and I've hot-plugged SCSI often enough that
I'd expect to have been bitten, but perhaps I was just lucky in that I
happened to have used tolerant hardware....

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