FAX

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What is FAX and why do I care

FAX is a way to access files on the grid, with both Athena and ROOT from anywhere. The added bonus over dCache is the hierarchy of redirectors. This means, if you ask for a file that is not on LRZ, the request will transparently be redirected to another storage site.

Setup

FAX can be set up from cvmfs like this

export ATLAS_LOCAL_ROOT_BASE=/cvmfs/atlas.cern.ch/repo/ATLASLocalRootBase
source ${ATLAS_LOCAL_ROOT_BASE}/user/atlasLocalSetup.sh
localSetupFAX

this will also find the nearest redirector (the server FAX asks first for files) based on your location. You can it overwrite it to LRZ (to which it defaults if you run it on the cluster, LRZ or anywhere in Garching really) with

export STORAGEPREFIX=root://xrootd-lrz-lmu.grid.lrz.de:1094/

Make sure you have a valid VOMS proxy

voms-proxy-init -voms atlas

now generate your file list from your dataset:

fax-get-gLFNs mc15_13TeV.341195.PowhegPythia8EvtGen_CT10_AZNLOCTEQ6L1_ggH125_mumu.merge.DAOD_HIGG3D1.e3945_s2608_s2183_r6765_r6282_p2419/

which looks like this and can directly plugged into Athena or ROOT, as long has you have a valid VOMS proxy

root://xrootd-lrz-lmu.grid.lrz.de:1094//atlas/rucio/mc15_13TeV:DAOD_HIGG3D1.06463414._000001.pool.root.1

Using FAX in your analysis

Have a VOMS proxy, and run over the file list you have just created.

If you use slurm, you will have to make that VOMS proxy available on every machine. When you create one, you will see output like this (the number will be different, in fact it is your user id)

Created proxy in /tmp/x509up_u12276.

Copy this file somewhere all your jobs can access it, like your home directory. Now each job must make that file known, and you will have access. To do that, simply set this env variable in each job.

export X509_USER_PROXY=/home/f/Friedrich.G.Hoenig/x509up_u12276

Enjoy your data.