DUNE xrootd

Documentation

dCache/xrootd

• A comprehensive review of dCache/xrootd. This document is quite relevant as it explains how dCache storage at FNAL is equipped with a "xroot door" so that it's exposed to external xrootd servers.

A pedestrian view on running a xrootd service

Disclaimer

The information below is not a reliable piece of documentation meaning it was obtained by experimentation with a small xrootd built from scratch in trying to overcome a modicum of obscurity that is present in the official xrootd docs. There are probably more scientific and proper ways to do things.

Starting a simple instance of xrootd service

There is more than way to start the xrootd service (see documentation). The most primitive way is to start the requisite daemon processes from the command line. A few details are given below.

Starting the xrootd daemon by itself is enough to serve data from a single node.

xrootd -c configFile.cfg /path/to/data &

In this case configFile.cfg contains the necessary configuration. Without it present, some simple defaults will be assumed but one cannot do anything remotely meaningful.

The "-b" option will start the process in the background by default, and the "-l" option can be used to specify the path to the log file (otherwise stderr will be assumed).

Likewise, if the "path to data" is not explicitely defined, xrootd will default to /tmp which might work for initial testing but isn't practical otherwise. Whether xrootd is running as expected can be tested by using the xrdcp client from any machine from which the server is accessible, e.g.

xrdcp myFile.txt root://serverIP//path/to/data

Clustering

In a clustered environment, you also need to start the cluster manager daemon, e.g.

xrootd -c configFile.cfg /path/to/data &
cmsd -c configFile.cfg /path/to/data &

The data in the cluster is exposed through the manager node, whose address is to be used in queries. Example:

xrdcp -f xroot://managerIP//my/path/foo local_foo

The file "foo" will be located and if it exists, will be copied to "local_foo" on the machine running the xrdcp client. Caveat: if multiple files exist in the system under the same path, the result (i.e. which one gets fetched) is random.

Configuration File

An example of a working configuration file suitable for a server node (not for the manager node):

all.role server
all.export /path/to/data
all.manager 192.168.0.191:3121
xrd.port 1094
acc.authdb /path/to/data/auth_file

In the example above the IP address for the manager needs to be set correctly, it's arbitrary in this sample.

authdb

The "authdb" bit is important, things mostly won't work without proper authorization (quite primitive in this case as it relies on a file with permissions). If all users are given access to all data, the content of the file can be as simple as

u * /path/to/data lr

Redirector

The redirector coordinates the function of the cluster. For example, it finds the data based on the path given by the clients such as xrdcp, without the client having to know which nodes contains this bit of data. A crude (but working) example of the redirector configuration:

all.manager managerIP:3121
all.role manager
xrd.port 3121
all.export /path/to/data
acc.authdb /path/to/data/auth_file

Note the port number. This is not the data port but the service port to used for communication inside the cluster (e.g. for metadata).

Of course the redirector itseld can also carry data, so configuration of the server might look like this:

all.manager managerIP:3121
all.role manager
xrd.port 1094
all.export /path/to/data
acc.authdb /path/to/data/auth_file

A crude way to initiate a node in this role might look like this

xrootd -c server.cfg /path/to/data &
cmsd -c redir.cfg /path/to/data &

xrdfs

File Info

Filesystem functionality. Example:

xrdfs managerIP ls -l /my/path
xrdfs managerIP ls -u /my/path

In the above the first item performs similarly to "ls -l" in Linux shell, the second prints URLs of the files.

The following command locates the path, i.e. returns the address(es) of the server(s) which physically hold(s) the path - can be multiple machines:

xrdfs managerIP locate /my/path

Adding the "-r" option will force the server to refresh, i.e. to do a fresh query. Otherwise, a cached result will be used if it exists.

The "stat" command provides info similar to "stat":

xrdfs managerIP stat /my/path

The "rm" command does what the name suggest, with the usual caveat that if same path is present on a few machines, the result will be arbitrary - one of the files will be deleted at a time.

Host Info

xrdfs hostIP query config role

Checksum

XRootD hosts can report checksums for files, with a few checksum algorithms available. To enable this on a host a special line needs to be added to the configuration file, for example:

xrootd.chksum md5

As usual, it is only necessary to query the redirector in order to get this info by the xrdfs client:

xrdfs managerIP query checksum /my/path/to/file

xrootd@BNL

Currently there is a small DUNE Cluster (for historical reason named "lbne cluster") at Brookhaven National Lab under the umbrella of RACF RHIC and ATLAS Computing Facility. The machines have names like lbne0001 etc. Xrootd software is deployed on all of these. To utilize it, the user needs to be authenticated with a X.509 certificate by the xrootd service and authorized to access it by system administrators (please contact Brett Viren or Maxim Potekhin for further information.

Once authorized on the site, the user will need the use the following commands to obtain the Grid proxy:

setenv GLOBUS_LOCATION /afs/rhic.bnl.gov/@sys/opt/vdt/globus
source $GLOBUS_LOCATION/etc/globus-user-env.csh
grid-proxy-init

...and enter the passphrase as required. This will make sure the user can be authenticated to the xrootd service is allowed to use it.

The following is an example of a shell command that will transport a single file from FNAL to BNL:

xrdcp root://lbnelrd.rcf.bnl.gov//lbne/mc/lbne/simulated/001/singleparticle_antimu_20140801_Simulation1.root \
/tmp/singleparticle_antimu_20140801_Simulation1.root

Possible xrootd architecture for medium term

The idea behind the architecture proposed here is to achieve federation of storage and access to data across a few data centers (e.g. national labs) with modest amount of effort and resources. In this approach, this is effectively achieved by using a "global redirector" which allows xrootd services to locate a particular piece of data within the federation.

Misc

For Xrootd we can have global Xrootd paths like:

root://data.<tbd>.org/path/to/file.root

But, in the future we may want to serve data files on other protocols but in the same domain/namespace. Ie:

http://data.<tbd>.org/path/to/file.root

Since the two are on different ports this should be okay.

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