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1750 redundant power supplies



If you've looked at our Dell 1750's, you've probably noticed that they
all have dual power plugs on the back.  "Ah-hah", you might think, "if
we plug both in, we get redundant power".  Or at least, that's
certainly what I thought.  Well, not quite.  Turns out the Dell 1750
actually does support dual hot swappable power supplies.
Unfortunately, somebody seems to have equipped ours with only *one* hot
swappable power supply per machine, perhaps because we didn't ask for
dual hot swappable supplies when we ordered our machines.  So, in fact,
only one of the plugs on the back of the machine works and the other
one does not, and "hot swappable" mainly means fewer screws for
repair purposes.

The good news is, our machines *do* have that nice cumfy hole for
another power supply.  The machine can be opened up even while "hot" by
removing the one screw visible on top of the case, then pressing the
large obvious "latch release" button on the front, then opening the
long thin front to back panel that the screw went through, there are 2
visible power supply bays.  The front bay holds PS1, and the empty back
bay holds PS2.  On the back panel, the outermost connector goes to PS2
(not even connected on nearly all of our machines), and the innermost
connector goes to PS1.  It's then possible to add a power supply (drops
in, slides towards the center connector), even while the machine is
live.  There are even status lights on the supplies so you can tell if
everything is kosher before unplugging anything.  Once both supplies
are in, it's possible to arbitrarily drop power on one or the other
supply without crashing the machine.  I actually tested this with one
of our machines (the future fear) so I'm pretty sure it all works just
fine.  If you do any of this, the only weirdness is that the blue light
on the front of the machine turns to flashing yellow.  I don't know of
any way to fix that short of rebooting the machine or installing the
probably evil dell management software, but it's completely harmless.

Extra power supplies
	Dell dps-312ab-a
appear to run as little as $21-$25 on ebay, or perhaps as much as $100
from a dealer.  In any case, I plan to ask Andrew to get at least 5 for
iaa (fear, surprise, ruthless, fanatical, replacement shabby).  Once
they're in, I expect to have small disturbing sessions with most of
these machines to install them.  (If we can get these installed before
the evil "no changes" period at the end of term that might make our
management happier...)

I can think of one other possibly more immediate payoff for this.  The
web folks have a whole rack of 1750's that I believe are on an
overloaded 20A circuit.  By borrowing just one extra 1750 power supply
and "walking" it down a rack, they could transfer the circuit these
machines are on potentially without any interruption in service.  It
will be a slightly nerve-racking affair, so I don't know if they'll
want to do this, but it's definitely an option.

				-Marcus