[Beowulf] WD green harddrive, greenwashing?

Mark Hahn hahn at mcmaster.ca
Sat Aug 4 14:15:36 PDT 2007

> green wagon. Anyone care to comment about his new WD hardrive?

I'd sure rather vendors push some modest power-savings than, say, 
putting in little windows so you can see the heads moving ;)

the power dissipation is 7.5/4.0/.3 (active/idle/standby) vs 8.77/8.4/.97 for
a similar less green disk.  obviously, you save more if you can spindown into
standby, but that (like the idle-but-spinning savings) assumes you do not
have a continuous load.

providing flash on disk will certainly help as well, since it will
mean that if you have standby configured, the OS can commit many writes
without causing the disk to spinup.  there is or was a tunable in linux
that caused the kernel to batch up IOs more agressively ("laptop mode"),
which would help as well.  heck, how many disks in your cluster could 
use noatime when mounting local disks - that would probably let quite 
a few disks spindown for extended periods...

ultimately, though, disks are minor factors in the power equation.
if I add up all the disks in my favorite cluster, it's still 
only 16KW or 5% of the total dissipation.

there are lots of things you can do to improve the greenness of your cluster.
atime and hdparm -S might help if your nodes have local disks.  getting rid 
of local disks (net-booting) is probably a good option if you can possibly
get away with it - both reliability and dissipation.  requiring right-sized
and efficient power supplies could easily save more than disks, though
(10% savings on 400W ~= removing 4 disks).  lots of systems have demonstrated
that you can sometimes trade off clocks for power (but this is fundamentally
dependent on the coupled-ness of your workload.)

I'd love to see a more holistic approach that considers nonstandard cooling
approaches, as well.  when we built our machineroom, the design types were 
basically unwilling to consider anything even slightly risky (our ground 
temperature here is basically perfect for chilling water all year round.)
I wonder if anyone has demonstrated using outside air to cool machinerooms 
(directly).  the stumbling block for that seems to be that people think
computers are terribly delicate (carefully controlled humidity, for instance)
and don't want to take any risks.  most specs I see are merely 
"5-95% non-condensing", which is pretty damn broad.

in the big picture, the problem is actually that power is too cheap ;)
the 4W savings for one WD GP disk in a desktop here is $1.2/year...

regards, mark hahn.

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