Xrootd is a service implemented in ROOT that allows the user to remotely read data located at various CMS sites. The protocol details can be found at the following TWiki page:
Once you have a valid grid proxy (obtained by doing
voms-proxy-init --voms cms), you can read a file on a remote site by using a ROOT command like.
TFile *f = TFile::Open("root://cmsxrootd.fnal.gov//store/mc/path/to/file");
root://cmsxrootd.fnal.gov/ is known as the redirector. In this case, it is the Fermilab redirector. There are also
root://xrootd-cms.infn.it/ for Europe regional redirector, and
root://cms-xrd-global.cern.ch/ for global redirector.
/store/data/... is known as the Logical File Name (LFN), which is the global identifier of an official CMS dataset.
You can also access non-official files stored under
/store/user/username/... at any CMS storage element that provides Xrootd service.
To download the file, you can use
xrdcp root://cmsxrootd.fnal.gov//store/mc/path/to/file /some/local/path
In case your grid proxy is not recognized, check if the environment variable
$X509_USER_PROXY is set. If not, set it by doing:
export X509_USER_PROXY=/tmp/x509up_u`id -u`
export X509_USER_PROXY=`voms-proxy-info -path`
To debug what’s wrong with Xrootd, do