[Beowulf] RE: Capitalization Rates - How often should you replace a cluster? (resent - 1st sending wasn't posted ).

Robert G. Brown rgb at phy.duke.edu
Sun Jan 18 20:40:35 EST 2009


On Sun, 18 Jan 2009, Greg Lindahl wrote:

> On Fri, Jan 16, 2009 at 08:10:43PM -0500, Mark Hahn wrote:
>>> The question was raised as "When should all these servers be upgraded or replaced again?"
>>
>> 3-5 years, IMO.  if you replace hardware in <3 years, you're obviously
>> burning money.
>
> After factoring in rent and utilities, my replacement time is < 3
> years. So what's obvious to you doesn't seem very obvious to me!

Right.  There are rules of thumb, but they are based on assumptions, and
if the assumptions aren't applicable they will lead to non-optimal
behavior.  The right rule is "do the cost-benefit analysis and act
according to what it tells you".  Which will, in fact, often lead to 3-5
year replacement cycle.  But additional costs along the way alter the
landscape, shifting to different replacement cycles.

To do the computation correctly, you have to include all sorts of
marginal costs and benefits.  For example, sometimes there are nonlinear
benefits to finishing work faster, which favors shorter cycles.
Sometimes there are nonlinear costs (or higher than normal linear
costs, which ALSO favors shorter cycles.

On the other hand, if somebody else pays for the power, and you have no
source of money to buy replacement nodes, you run nodes until they die.
All the "usual" CBAs assume a constant flow of support moneys for new
nodes, and that is not always the case.

   rgb

>
> -- greg
>
>
>
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Robert G. Brown                            Phone(cell): 1-919-280-8443
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