[Beowulf] Newbie Question: Racks versus boxes and good rack solutions for commodity hardware

[Geoff Jacobs]

Robert G. Brown wrote:
> On Sat, 13 Dec 2008, arjuna wrote:
> 
>> A simple question though...Aluminum plates are used because aluminum
>> is does
>> not conduct electricity. Is this correct?
> 
> Aluminum is an EXCELLENT conductor of electricity, one of the best!
> Basically all metals conduct electricity.  When you mount the
> motherboards you MUST take care to use spacers in the right places
> (under the holes for mounting screws on the motherboards, usually) to
> keep the solder traces of the motherboard from shorting out!
> 
> Your question makes me very worried on your behalf.  Electricity is
> quite dangerous, and in general messing with it should be avoided by
> anyone that does not already know things like this.  In India, with 240
> VAC as standard power, this is especially true.  True, the power
> supplied to the motherboards is in several voltages 12V and under, but
> believe it or not you can kill yourself with 12V, and starting a fire
> with 12V is even easier.

You can actually do a good job of TIG welding with 12V.

> I would >>strongly<< suggest that you find a friend with some electrical
> engineering experience, or read extensively on electricity and
> electrical safety before attempting any sort of motherboard mount.
> Mark's suggestion of hot melt glue, for example, is predicated on your
> PRESUMED knowledge that cookie sheets or aluminum sheets are
> conductors, that the motherboard has many traces carrying current, and
> that when you mount the motherboard you must take great care to ensure
> that current-carrying traces CANNOT come in contact with metal.
> 
> The reasons aluminum plates are suggested are a) it's cheap; b) it's
> easily drilled/tapped for screws; c) it's fireproof AS LONG AS YOU DON'T
> GET IT TOO HOT (heaven help you if you ever do start it on fire, as it
> then burns like thermite -- oh wait, thermite IS aluminum plus iron
> oxide); d) it reflects/traps EM radiation.

Ask the Royal Navy about fireproof aluminum.

> Wood would be just as good except for the fireproof bit (a big one,
> though -- don't use wood) and the EM reflecting part.
> 
> The aluminum plates should probably all be grounded back to a common
> ground.  The common ground should NOT be a current carrying neutral --
> I'm not an expert on 240 VAC as distributed in India and hesitate to
> advise you on where/how to safely ground them.  You should probably read
> about "ground loops" before you mess with any of this.

Commodity ATX power supplies will have a grounded frame. Mounting the
power supply to the pan will work quite well.

> Seriously, this is dangerous and you can hurt yourself or others if you
> don't know what you are doing.  You need to take the time to learn to
> the point where you KNOW how electricity works and what a conductor is
> vs an insulator and what electrical codes are and WHY they are what they
> are before you attempt to work with bare motherboards and power
> supplies.  It is possible to kill yourself with a nine volt transistor
> radio battery (believe it or not) although you have to work a bit to do
> so.  It is a lot easier with 12V, and even if you don't start a fire,
> you will almost certainly blow your motherboard/CPU/memory and power
> supply if you short out 12V in the wrong place.

Yes, it is possible to kill yourself with low voltage. You have to
really work at it and/or be unlucky, but it can be done. A DC resistance
from leg to arm of 100 ohms or so is hard to achieve. Stabbing oneself
with electrified needles, for starters.

>> Also for future reference, I saw a reference to dc-dc converters for
>> power
>> supply. Is it possible to use motherboards that do not guzzle electricity
>> and generate a lot of heat and are yet powerful. It seems that not
>> much more
>> is needed that motherboards, CPUs, memory, harddrives and an ethernet
>> card.
>> For a low energy system, has any one explored ultra low energy
>> consuming and
>> heat generating power solutions that maybe use low wattage DC?
> 
> The minimum power requirements are dictated by your choice of
> motherboard, CPU, memory, and peripherals.  Period.  They require
> several voltages to be delivered into standardized connectors from a
> supply capable of providing sufficient power at those voltages.  Again,
> it is clear from your question that you don't understand what power is
> or the thermodynamics of supplying it, and you should work on learning
> this (where GIYF).  As I noted in a previous reply, typical motherboard
> draws are going to be in the 100W to 300+W loaded, and either you
> provide this or the system fails to work.  To provide 100W to the
> motherboard, your power supply will need to draw 20-40% more than this,
> lost in the conversion from 120 VAC or 240 VAC to the power provided to
> the motherboard and peripherals.  Again, you have no choice here.
> 
> The places you do have a choice are:
> 
>   a) Buying motherboards etc with lower power requirements.  If you are
> using recycled systems, you use what you've got, but when you buy in the
> future you have some choice here.  However, you need to be aware of what
> you are optimizing!  One way to save power is to run at lower clock, for
> example -- there is a tradeoff between power drawn and speed.  But
> slower systems just mean you draw lower power for longer, and you may
> well pay about the same for the net energy required for a computation!
> You need to optimize average draw under load times the time required to
> complete a computation, not just "power", weighted with how fast you
> want your computations to complete and your budget.
> 
>   b) You have a LIMITED amount of choice in power supplies.  That's the
> 20-40% indicated above.  A cheap power supply or one that is incorrectly
> sized relative to the load is more likely to waste a lot of power as
> heat operating at baseline and be on the high end of the power draw
> required to operate a motherboard (relatively inefficient).  A more
> expensive one (correctly sized for the application) will waste less
> energy as heat providing the NECESSARY power for your system.
> 
> That is, you don't have a lot of choice when getting started -- you're
> probably best off just taking the power supplies out of the tower cases
> of your existing systems and using them (or better, just using a small
> stack of towers without remounting them until you see how clustering
> works for you, which is safe AND effective).  When you have done some
> more research and learned about electricity, power supplies, and so on
> using a mix of Google/web, books, and maybe a friend who works with
> electricity and is familiar with power distribution and code
> requirements (if any) in New Delhi, THEN on your SECOND pass you can
> move on to a racked cluster with custom power supplies matched to
> specific "efficient" motherboards.
> 
>    rgb

Although this list is quite liberal, we should be fair to Donald Becker
and Co. in pointing out that most of the questions you have, here, are
general computer hardware/electrical questions. They are best dealt with
aside from this list. However, we'll make sure you have a good place to
start.

Buff up on basic electronics, both theoretical and practical. Start with
a good textbook like Grob, "Basic Electronics" and something like an
electronics projects kit. We used to be able to buy them at Radio Shack.

If you just want to cut to the chase and assemble some computers, GIYF.
Look for "build your own computer". For example, I have included one of
the first links available. It appears to be fairly thorough.
http://www.pcmech.com/byopc/

Once you're at the stage where you are comfortable with computer
hardware, at least, and perhaps electronics in general, then you will be
prepared to build a Beowulf. We will be happy to help with any questions
at this stage, buy please check google first always, as sometimes the
answer is already out there.

-- 
Geoffrey D. Jacobs