[Beowulf] Cooling vs HW replacement

[Mark Hahn]

> > Use a SAN/NAS (nfs) and keep the disks in a separate room than the CPUs.
> > Disk drives generate a lot of heat, and compared to on board components
> > don't really need cooling, circulated air should largely cover them.
> 
> This is an oxymoron.  If disks generate a lot of heat, then that heat
> needs to be removed.  If you have a small room stuffed with hundreds
> of those disks, all that heat has to go somewhere...

not only is it an oxymoron, but it's also not true ;)
disks do not generate a lot of heat.

aside: Maxtor, in particular, has gradually stopped putting ANY 
useful data into their so-called datasheets.  they used to at least
give you idle and seek current, for instance.  now it's only idle,
which seems deliberately perverse ;)

Seagate, on the other hand, has improved their documentation - 
you can actually get 100 pages of spec on the Cheetah, for instance.
the quality reminds me of thorough specs that IBM used to publish 
about their disks.  from that doc 12W idle, peak 18W active.
that's 10K, and a 2G FC model, which is noticably higher than a SCSI
model, and a lot higher than a SATA model, especially 7.2K.

18W for a large, maximally hot disk.  disks are usually packed at densities
of around 4/U, so a full rack (no space for controllers, etc) is about 3.1KW.
as compared to maybe 14KW for a fairly agressive compute rack.  (yes, I know
there are some vendors who are at least talking about 8-10 disks/U...)

more realistically, just use nice modern SATA disks at half that power,
and twice the GB/disk (or 1/4 the KW/TB).  and as your storage needs grow,
the importance of power-savings increases, since you are less likley to keep
2 TB busy than 1 TB.  for this particular Cheetah, it looks like you go down
to under 5W if you spin down.

oh, cool!  HGST is still doing very nice specs, 271 pages for 7k400:
sleep		.7W
standby		1
low rpm idle	4.4/6.8/9.0 (depends on load/unload and "low rpm idle" mode
random r/w	13W

regards, mark hahn.