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CMS Xrootd Service

This document covers joining the CMS xrootd service based on the redirector xrootd.unl.edu. This was originally written for CMS HDFS sites, but any xrootd instance that: a Exports a compliant xrootd protocol a Is compatiable with a Scalla cmsd daemon for redirection. a Provides security acceptable to CMS (i.e., GSI-based authentication). The last two bullet points may limit the acceptable clients to only Scalla-based xrootd servers. We have integrated HDFS with Scalla xrootd by providing a custom integration module that implements a Scalla Xrootd OFS. Any storage element exporting a reasonably stable C API can write such a module.

Any storage element which is POSIX-compliant (Lustre, GPFS, etc) needs no customization - it can run the Scalla Xrootd daemon directly!

The rest of the document assumes you are going to run the Scalla Xrootd server on top of HDFS.

Installation

You must already have HDFS working and configured on the node. A FUSE mount is not needed; however, you should be able to use hadoop -put and hadoop -get to move files in and out of HDFS on the node. If the node is already a functioning HDFS GridFTP server, then it probably meets these requirements.

First, add the OSG-Hadoop repository to the node (if it is not there already):

if [ ! -e /etc/yum.repos.d/osg-hadoop.repo ]; then
  rpm -Uvh http://vdt.cs.wisc.edu/hadoop/osg-hadoop-1-2.el5.noarch.rpm
fi

Next, install the xrootd RPM from the unstable repo. This will add the xrootd user if it does not already exist - ROCKS users might want to create this user beforehand.

yum --enablerepo=hadoop-unstable install xrootd
The version should be at least 1.1-4.

If your CMS namespace is not truly trivial (i.e., if the CMS top-level directory in Hadoop is not /store), copy your storage.xml to /etc/xrootd/storage.xml. Make sure your storage.xml exports a hadoop protocol (which should provide the PFN relative to Hadoop; see Nebraska's TFC as inspiration if necessary).

Copy the template config file, /etc/xrootd/xrootd_sample.cfg to /etc/xrootd/xrootd.cfg. If your site requires storage.xml, uncomment (and possibly update) the oss.namelib line.

Finally, create a copy of the host certs to be xrootd service certs:

mkdir -p /etc/grid-security/xrd
cp /etc/grid-security/hostcert.pem /etc/grid-security/xrd/xrdcert.pem
cp /etc/grid-security/hostkey.pem /etc/grid-security/xrd/xrdkey.pem
chown xrootd: -R /etc/grid-security/xrd
chmod 400 /etc/grid-security/xrd/xrdkey.pem # Yes, 400 is required

Operating xrootd

There are two init services, xrootd and cmsd, which must both be working for the site to participate in the xrootd service:

service xrootd start
service cmsd start

Everything is controlled by a proper init script.

Log files are kept in /var/log/xrootd/{cmsd,xrootd}.log, and are auto-rotated.

After startup, the xrootd and cmsd daemons drop privilege to the xrootd user.

Port usage:

The following information is probably needed for sites with strict firewalls:
  • The xrootd server listens on TCP port 1094.
  • The cmsd server needs outgoing TCP port 1213 to xrootd.unl.edu.
  • Usage statistics are sent to xrootd.unl.edu on UDP ports 3333 and 3334.

Testing the install.

The newly installed server can be tested directly using:
xrdcp xroot://local_hostname.example.com//store/foo/bar /tmp/bar
You will need a grid certificate installed in your user account for the above to work

You can then see if your server is participating properly in the xrootd service by checking:

xrdcp root://xrootd.unl.edu//store/foo/bar /tmp/bar2
where /store/foo/bar is unique to your site

Known Issues:

  • In 1.1-4, you need to make sure the line "HADOOP_CONF_DIR=/etc/hadoop" is in /etc/sysconfig/hadoop
  • In 1.1-4, most sites will want to change the log level for xrootd.log to be less verbose. Change the ofs.trace line to read ofs.trace none.
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Topic revision: r2 - 2010-04-06 - BrianBockelman
 
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