Accessing CERN EOS from Liverpool HEP

Files stored on the CERN EOS service can be accessed directly from Liverpool HEP Centos 7 systems in two main ways: via the EOS client or via XROOTD.

The EOS client is recommended for light access eg listing files, copying small amounts of data. XROOTD access is recommended for heavy data access eg batch analysis jobs or bulk data copying.

Accessing via EOS Client

Using the local EOS client requires either a valid CERN kerberos token, or a VOMS proxy. Generally for full access the kerberos token works best. VOMS proxies only seem to work with some EOS areas.

Using system /eos mount

Centos7 interactive nodes (and maybe desktops) should have the EOS areas for experiments mounted already under /eos/EXPERIMENTNAME. To access this you just need your valid CERN kerberos token.

You need to enable the binding first with
  • eosfusebind
Files and directories should be accessible under /eos/ as normal. When you are finished remove the binding with
  • eosfusebind -u
The EOS user areas are also available under /eos/user. Follow the same procedure.

Using user /eos mount

Specific experiment EOS services can be mounted directly on a local directory. This seems to work better with VOMS proxies, but kerberos tokens are still recommended for full access.

Configure the service details with eg
  • export EOS_FUSE_MGM_ALIAS=eosatlas.cern.ch
  • export EOS_MGM_URL=root://eosatlas.cern.ch
Then mount the service on a local directory eg $HEPTMP/eos
  • mkdir $HEPTMP/eos
  • eos fuse mount $HEPTMP/eos
The EOS area for that experiment will now be available under $HEPTMP/eos. When you have finished accessing files unmount the area with
  • eos fuse umount $HEPTMP/eos

Accessing via XROOTD

Files can be accessed from CERN EOS without any local clients or mounts by directly accessing the files using the XROOTD protocol. ROOT should support this protocol natively so data can be read straight from EOS without having to copy files locally first.

A typical XROOTD URL for a file on eg the ATLAS EOS service such as /eos/atlas/atlascerngroupdisk/somedir/somefile.root would be
  • root://eosatlas.cern.ch//eos/atlas/atlascerngroupdisk/somedir/somefile.root
This URL can be used directly in ROOT.

This is the recommended method to use for batch jobs or interactive ROOT sessions with heavy data analysis as the access should be faster than using the EOS client and requires no local configuration (perfect for batch jobs).

Files can also be copied locally with the xrdcp tool eg
  • xrdcp root://eosatlas.cern.ch//eos/atlas/atlascerngroupdisk/somedir/somefile.root /my/local/dir/somefile.root
Please only use this sparingly, direct access is usually efficient enough and saves consuming local resources.

Other experiments or areas will have their own EOS services (eg eoslhcb.cern.ch for LHCB or eosuser.cern.ch for the User areas).
Topic revision: r2 - 21 Feb 2019, JohnBland
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