[Beowulf] /dev/random entropy on stateless/headless nodes

Lux, Jim (337C) james.p.lux at jpl.nasa.gov
Mon Feb 28 09:24:24 EST 2011

foot per ns in free space.  60-80% of that in a transmission line, depending on sqrt(LC)  (mostly depends on dielectric.. solid vs foam vs air), except for some exotic delay line coax which has a center conductor wound as a spiral, so the L/unit length is really big.

so more like 20-25 cm/nanosecond.  10cm/ns would be a very slow line
From: beowulf-bounces at beowulf.org [beowulf-bounces at beowulf.org] On Behalf Of Hearns, John [john.hearns at mclaren.com]
Sent: Monday, February 28, 2011 02:27
To: beowulf at beowulf.org
Subject: Re: [Beowulf] /dev/random entropy on stateless/headless nodes

So (from the wiki) piping dumb data into /dev/random is harmless since the entropy measure wouldn't be fooled, so then yes, just anyone can pipe some bytes in anytime. So yeah, the rtdsc, I just meant my 9 digits of nanoseconds as something easy to try at boot time, and shuffling that with the MAC is a good idea. (Since a light-nanosecond is about what 10 cm? the lengths of cables in the server room would be enough to give every node different boot times, in nanoseconds, right? or no, because your cables are all standard lengths, but coiled as needed?).

Me Sir! Please! Me Sir!

As a cub high energy physicist, in the ‘counting room’ for an experiment one day, my supervisor was discussing coaxial delay lines with me.
Light travels at a foot per nanosecond, so that enables you to choose appropriate lengths of coax delay lines for coincidence counting experiments.

Sigh – I guess we have gone all metric since then with 10cm per nanosecond!

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