[Beowulf]Infrastruture planning for small HPC 40/100 gigabyet eyhernet or Infiniband?

Perry E. Metzger perry at piermont.com
Tue Jul 29 09:33:16 PDT 2008

Eugen Leitl <eugen at leitl.org> writes:
> On Sun, Jul 27, 2008 at 07:19:56PM -0700, Jim Lux wrote:
>> bear in mind that ordinary ethernet both coax and twisted pair is  
>> galvanically isolated.
> This is strange, because I've seen (small) sparks and received (mild)
> shocks from both, in two different locations. 

Ground loops are a real phenomenon in UTP Ethernet. For example,
*NEVER* run UTP between buildings. If the grounds in the two buildings
are at a different relative potential, and they often are, very bad
things can happen.

The building complex I live in ran Cat 5 between buildings in
underground ducts. They were very surprised when lightning strikes
some distance away regularly blew out the switches. Changing to fiber
eliminated the problem, of course.

> In any case I'll have an electrician diagnose the problem.

If you're seeing sparks, as you say, I suspect you do indeed have an
AC supply problem. Ground loop, or something worse. (The Electrical
Wiring FAQ describes several problems that qualify as "worse"...)

Perry E. Metzger		perry at piermont.com

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