FAX end-user Tutorial

Introduction

A step-by-step how-to tutorial for using FAX as an end-user. The tutorial examples can be downloaded here. If something does not work or you have any questions, please contact atlas-adc-fax-operations@cernNOSPAMNOSPAMPLEASE.ch.

Setup environment

Setting up the FAX environment is done using a package from the localSetupFAX (more here):

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

localSetupFAX --rootVersion=current-SL6

A FAX access endpoint is automatically setup for you. You can find it in the environment variable STORAGEPREFIX. Create the usual grid proxy:

    voms-proxy-init -voms atlas
    

Check availability of the dataset or dataset container in FAX

Use fax-is-dataset-covered, available upon invoking localSetupFAX. It takes the dataset or dataset container name as an argument. For each input dataset, it will print number of FAX endpoints containing full and incomplete replicas. Example (user.ilijav.HCtest.1 is a test dataset that exists at several sites):

   fax-is-dataset-covered user.ivukotic:user.ilijav.HCtest.1
   

Copy a dataset to a local storage

Use fax-get to copy a dataset from FAX to local storage. Command supports multiple streams, retries, partial dataset copy, skipping non-root files, timeouts and more. Example:

 
   fax-get user.ivukotic:user.ilijav.HCtest.1
   

Find global logical file names (gLFNs)

Currently there are two conventions for the global logical file namespace used by FAX:

• Use tool fax-get-gLFNs:

  fax-get-gLFNs user.ivukotic:user.ilijav.HCtest.1  > my_list_of_gLFNS.txt 

Copy a file from FAX to local disk

• With the environment setup above:

  xrdcp $STORAGEPREFIX/atlas/rucio/user.ivukotic:group.test.hc.NTUP_SMWZ.root /tmp/myLocalCopy.root 

Open and inspect a file with ROOT

• Open and list the file contents with ROOT.
• Then:

     root -l 
     TFile *f = TFile::Open("root://fax.mwt2.org//atlas/rucio/user.ivukotic:group.test.hc.NTUP_SMWZ.root"); 
     if(f) f->ls();
     .q 

Using FAX from a prun job

Additional examples

• Listing number of events
• Reading-writing a file
• Simple SkimSlim example
• SkimSlim example from T3
• A large SkimSlim example from T3
• Analysis-like example

FAX DO's & DONT's

• Always, always, always use TTreeCache. Without it performance will be very bad. It should be used even if you don't use FAX.
• If using TChain, don't create TFiles and give them to TChain but directly give filenames to TChain. TChain will make sure files can be opened.
• Messages like this

  Xrd: XrdClientMessage::ReadRaw: Failed to read header

  can be safely ignored and will be removed in future versions of xrootd.

Responsible: IlijaVukotic
Last reviewed by: Never reviewed