[Beowulf] SSD caching for parallel filesystems

Lux, Jim (337C) james.p.lux at jpl.nasa.gov
Mon Feb 11 16:13:36 PST 2013

Not simulations..

Streaming the data out to a Digital to Analog converter..

Basic box works like this:
Attitude and guidance simulator gives a parallel bit word that indicates range and doppler and target type.. that's used to set a timer and frequency offset
Radar gives a sync pulse to the target simulator that indicates that the pulse will be transmitted in a short time, and a parallel word telling it which kind of pulse it's going to transmit.
Target simulator points to the correct pulse in the bulk storage which holds a bunch of simulated returns based on target type and pulse type  (e.g. a 4 ns pulse will have a different return than a 1ms long pulse)
Bulk store starts streaming the pulse into a FIFO.  A countdown timer for range delay triggers the output of the FIFO to a DAC which generates the actual pulse.

So you don't have to simulate the returns in real time.. .they're done off line in non-real time, and all the target box does is play back the correct return at the correct time and Doppler.

You could use a lot less sample returns and just adjust the amplitude and phase, however, you'd like to simulate things like a lateral translation across the edge of a rocky area to a flat sandy area: the former has a much more ragged return that lasts longer, while the latter is a more specular reflection and narrower.

If the beam is significantly tilted relative to the surface normal, the return is much longer in time than if it's pointing straight down, because the "spot" that is illuminated has significant range extent in the former case.

This is why you wind up needing lots of different potential returns for the same length transmitted pulse.

And that's for non-coded pulses.  Throw in coding on the radar pulse (chirps or PN codes) and it gets even more interesting.

One might even take a sample radar out to a test site in the desert (which looks an awful lot like the surface of Mars, at least to a radar) and record actual returns from different types of terrain.

I don't recall the costs, but in any case, a more important design aspect is getting it to work at all, rather than driving the price down to the minimum. 

I was responding to your question asking for an example of something needing high bandwidth and small storage, but that couldn't be adequately addressed by just buying a ton of conventional RAM.

These systems don't have a CPU.. it's just disk drive, FIFO, data converter.

I suspect digital video recorders are another application.. very similar kind of usage.. You stream raw video in at some Gbytes/second and just dump it to the drive(s). digital motion picture projectors at 4K resolution are probably another example.  Although there, you are looking at fairly large data sets.

The RED cameras stream to flash, SSD or conventional drives, for instance.  9Megapixels/frame*30 fps is 270 megapixels/second. 
The newer EPIC cameras do 31.8Mpix/frame * 96 fps...    I think they run about 400-500 Mbyte/sec to a SSD "magazine"
The film business is used to interchanging magazines. A typical 35mm cine camera has 400 ft and 1000ft magazines.  At the usual 24fps, that works out to about 1 foot/second, so 400 or 1000 seconds of shooting time.  A comparable RED/EPIC magazine, then, probably holds half a terabyte or so.  

Here you go.. a 4 pack of 256GB drives for $9100...

256GB is probably comparable to a 400 foot magazine at the highest resolution.

Jim Lux

-----Original Message-----
From: beowulf-bounces at beowulf.org [mailto:beowulf-bounces at beowulf.org] On Behalf Of Vincent Diepeveen
Sent: Monday, February 11, 2013 3:30 PM
To: beowulf at beowulf.org List
Subject: Re: [Beowulf] SSD caching for parallel filesystems

Ah you're doing the simulations a lot slower :) No big deal. You get what you pay for :)

You're speaking about 1 box here or so?

What size SSD array size is in that box, what bandwidth does it deliver to your CPU's and what price did you buy it for Jim?
Then we can compare it with a harddrive raid array :)

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