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Re: (OT) Storage (8*IDE HDs) any experiences?

On Wed, May 02, 2001 at 03:13:34PM -0500, Rich Puhek wrote:
> I agree with Matthew in that there _is_ a reason to share the load,
> actually a few that I can think of. Let's say you have a pair of 300W
> supplies on a box that draws 250W at rest. Rather than let one supply
> crank along at 250W, let's let both supplies run at about 125W. That
> way, both supplies will run cooler (Depending on the supply design, the
> supply may actually have slightly lower efficiency at the lower load
> factor, but that's a trade off we can live with). 

IANAEE/Physicist and it's been (dear Goddess, has it been this long?)
over 15 years since school, so my recollection is a bit fuzzy... 
Lower efficiency at lower load will probably mean more energy is 
dissipated as heat. So 2 PSUs * 125W ea will generate more heat than 
1 PSU * 250W. Depending on box design this may cause CPU/disks to 
operate at higher temperatures, reducing their lifespan. In fact,
there will be some point at which each individual PSU will run 
just as hot as if it handled all the load on its own (you can be sure
your box will draw exactly that much load, thank you Mr Murphy).

I'm not arguing that this is the case, I'm saying that this kind of
argument can be twisted and turned any which way you like. 

I had mid-80s AT PSUs. They'd still be working if I didn't have to
move house and throw all that junk out. If a PSU lasts for 15 years, 
will 2 load-sharing PSUs last 30 years? Do I care? (Will I last 30 
more years?) I know that CPU will maybe last 1/5th of that, disks 
maybe 1/3rd. So what is it I'm going to achieve by setting up 
load-balancing PSUs?

... Also consider what
> happens if the load was near the capacity of a single supply, and spiked
> over the capacity. If we were using the second supply as a "backup" to
> only be switched in if the primary failed, how would that be handled?

Well if you mean some piece of hardware suddently decides to draw
$BIGNUM times its normal current, the PSU will die. Depending on the 
design, there's a circuit somewhere (eg. on the backplane) that does 
the  appropriate magic and switches the second PSU on. Of course it'll 
die very soon too, unless the FPOH in question magically fixed itself 
in the meantime.

Sometimes the magic fails -- I remember the look on my boss's face when 
he pulled a hot-swappable PSU out of a live swerver, and the box went 
down. Oops. (Only happened once; we later tried to reproduce the problem, 
quite unsuccessfully: PSUs switched over like a charm, every bloody time.
Surprise, surprise.)

E-mail dmaziuk at bmrb dot wisc dot edu (@work) or at crosswinds dot net (@home)
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