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A false good idea


oles@ovh.net
09-03-2009, 10:54 AM
Hello,
While working on the future version of RPS (with concentration
1000 RPS / Bay), we are studying the possibility of feeding
RPS motherboards with PoE (Power Over Ethernet). Basically,
instead of having connected to the power outlet, electrical
power is provided directly by the PoE switch. The "hard"
system reboot is automatically integrated (just cut and
turn on the Ethernet port on the switch and your RPS restarts).
But above all it simplifies everything: in the bay there is
basically the switch and the RPS, and one single cable into
the RPS: the RJ-45. As there is no disks in the RPS and
the motherboard is low consumption: 2W when idle and
7-10W in operation. Everything bounds.

Apart from the cost of an infrastructure like that (PoE switches are
expensive), there are problems of maximum power delivered
through a port. The telephone standards allow delivery 48V
(less than 50V for human security without fuses) but
RJ45 cables do not allow more than 0.3A and thus ultimately
everything is clamped at about 20W/port. And
therefore such a facility would operate with only one type
motherboard but not all. So it would need to prepare a
converter in the RPS side to make sure that the RJ-45
provides enough electrical power to the motherboard.
Right now, PoE motherboards don't exists at all. Then
it's a matter of adapting the voltage from PoE to the
voltage of the mother board. So, even though the idea
sounds very good, there should be much debate still between
those who build motherboardsand switches to get to use
all these devices together and yet, it would be in very particular RPS.
Maybe it will be in the future, in 5 years, when we'll have 1W
for a 12 cores CPU of the sort we have today.....

So that's it. It was just to say that sometimes a good idea is
not necessarily very nice on arrival. But hey, that's part of
risks in trying to reinvent the wheel: to make a wheel that
meets our very specific needs.

Sincerely,
Octave