Racks vs. pile of PCs

[David Mathog]

Racks sure look nice and there is no question that they
are space efficient, but I'm really starting to wonder if
they are such a great idea for a smallish cluster (<=20 nodes)
in those situations where there is enough space for a
classic pile of PCs.   I mean, what other advantages do they
have besides those two to offset their many disadvantages?

Racks better than piles:

1.  Space efficiency.
2.  Aesthetics (racks look cool)

Piles better than racks (these are not orthogonal):

1.  Internal space constraints
2.  CPU/motherboard Cooling.  This follows from [1].
3.  Motherboard/CPU options.  This follows from [1]. 
    With a few exceptions most motherboard/CPU combinations
    will fit into a standard ATX case -  good luck getting
    a 2.4 Ghz P4 into a 1U.
4.  Initial purchase price for equivalent performance.
5.  Maintenance costs (rack parts tend to be nonstandard
    and expensive to replace, for instance, 1U power supplies).

Other factors?

I estimate that for a small cluster (<1 rack's worth of equipment) with
node guts (mobo,CPU,disk,ram) costing <= $1200 the racked version
will cost at least 20-30% more than the piled version.  So if a piled
20 node cluster costs $24000, the equivalent racked version will
be at least $30000.   $6000 seems a lot to pay for no extra performance.
If the "guts" were much more expensive the additional rack costs would,
in theory be a lower percentage.  In practice, it is my impression that
the ratio is no lower because the vendors charge even more for the 
racked versions of high performance nodes.

Comments? 

Regards,

David Mathog
mathog at caltech.edu
Manager, Sequence Analysis Facility, Biology Division, Caltech