Petabits/sec, and the like

John Hearns john.hearns at clustervision.com
Thu Nov 6 04:23:50 EST 2003


On Thu, 6 Nov 2003, Chris Samuel wrote:

> To take an example, I know of a local group doing work at CERN. Apparently 
> they are participating in experiments that generate around 1TB a day of data 
> (no idea if that's compressed/uncompressed or how compressible it would be).
> 
I THINK (though don't quote me) that this is the raw data rate.
What happens in an HEP experiment is that raw data comes from the
detector.
It is passed through three levels of trigger processors, from
a very simple (are 1st level inside the detector at LHC???) to a third
level, which is run on PCs.

I guess this 1TB rate is the raw event rate after the level 3 trigger.
The data is then sent to a reconstruction farm, where the raw levels
are combined into tracks and energy deposits, using the physical 
data and calibrations of the detector.

The physicists then work on the resulting DST - data summary tape,
which is much less data than the raw data.

I'm not sure of the plans for processing raw data at LHC - 
maybe all is processed at the main site, maybe som is shipped off
to the Tier 1 centres. I really don't know the answer here.

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