[Beowulf] petabyte for $117k

Joe Landman landman at scalableinformatics.com
Wed Sep 2 04:28:24 PDT 2009

Bill Broadley wrote:

> The lid screws down to apply pressure to a piece of foam.  Foam presses down
> on 45 drives.  5 drives (1.4 lb each) sit on each port multipliers.  6 nylon
> mounts support each multiplier supporting 7 pounds of drives.  Seems like the
> damping from the nylon mounts would be minimal under that much pressure.  So
> on the bottom of the case you have 63 pounds of drives, a significant fraction
> of which is rotating mass.

... Which is being driven at multiple points by a 120Hz signal 
(7200RPM).  It seems like a basic physics calculation to get the 
resulting eigen-modes and eigen-frequencies.  Looking at the design, I 
was concerned about the nylon standoffs (high frequency coupling, 
including octaves of 120Hz) coupling enough vibration into the units.

> I wonder if the port multipliers are really designed to actually support the
> drives.  Doesn't seem like the SATA power/data connects I've seen are designed
> for that.

Technically, they should not be used for structural support.  For 
supporting cables?  Sure.

> Not to mention it's hard to imagine the typical flexible thin sheet metal lid
> applying even pressure to 45 drives through the foam.  Seems like most of the
> pressure would be on the drives on the outside edge, leaving the inside drives
> relatively undamped.
> My experience is that nylon mounts don't help much.  Sure poor manufacturing
> tolerances, and near zero load/tension often prevent things like fans from
> tightly coupling with a chassis.  But to decouple drive vibration from a
> chassis seems to require something much more aggressive.  Something like a
> very soft/gooey rubber with a fair bit of travel and give under minimal
> pressure (4-6 ounces).

Yeah.  Looking at this unit, the big issue looks to be vibration.  In 
which case, you probably want better (more vibration tolerant) drives 
than the ones spec'ed.  And a better mounting design.

Joseph Landman, Ph.D
Founder and CEO
Scalable Informatics, Inc.
email: landman at scalableinformatics.com
web  : http://scalableinformatics.com
phone: +1 734 786 8423 x121
fax  : +1 866 888 3112
cell : +1 734 612 4615

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