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Installing the CMS XCache

This document describes how to install a CMS XCache. This service allows a site or regional network to cache data frequently used by the CMS experiment, reducing data transfer over the wide-area network and decreasing access latency. The are two types of installations described in this document: single or multinode cache. The difference might be based on the total disk that your cache needs.

Before Starting

Before starting the installation process, consider the following requirements:

  • Operating system: A RHEL 7 or compatible operating systems.
  • User IDs: If they do not exist already, the installation will create the Linux user IDs xrootd
  • Host certificate: Required for client authentication and authentication with CMS VOMS Server See our documentation for instructions on how to request and install host certificates.
  • Network ports: The cache service requires the following ports open:
    • Inbound TCP port 1094 for file access via the XRootD protocol
    • Outbound UDP port 9930 for reporting to and for monitoring
  • Hardware requirements: We recommend that a cache has at least 10Gbps connectivity, 100TB of disk space for the whole cache (can be divided among several caches), and 8GB of RAM.

As with all OSG software installations, there are some one-time steps to prepare in advance:

Installing the Cache

The CMS XCache ROM software consists of an XRootD server with special configuration and supporting services. To simplify installation, OSG provides convenience RPMs that install all required packages with a single command:

[email protected] # yum install cms-xcache

Configuring the Cache

First, you must create a "cache directory", which will be used to store downloaded files. By default this is /mnt/stash. We recommend using a separate file system for the cache directory, with at least 1 TB of storage available.


The cache directory must be writable by the xrootd:xrootd user and group.

The cms-xcache package provides default configuration files in /etc/xrootd/xrootd-cms-xcache.cfg and /etc/xrootd/config.d/. Administrators may provide additional configuration by placing files in /etc/xrootd/config.d/1*.cfg (for files that need to be processed BEFORE the OSG configuration) or /etc/xrootd/config.d/9*.cfg (for files that need to be processed AFTER the OSG configuration).

You must configure every variable in /etc/xrootd/10-common-site-local.cfg.

The mandatory variables to configure are:

  • set rootdir = /mnt/stash: the mounted filesystem path to export. This document refers to this as /mnt/stash.
  • set resourcename = YOUR_RESOURCE_NAME: the resource name registered with the OSG for example ("T2_US_UCSD")


XRootD can manage a set of independent disk for the cache. So you can modify file 90-cms-xcache-disks.cfg and add the disks there then rootdir just becomes a place to hold symlinks.

Ensure the xrootd service has a certificate

The service will need a certificate for reporting and to authenticate to CMS AAA. The easiest solution for this is to use your host certificate and key as follows:

  1. Copy the host certificate to /etc/grid-security/xrd/xrd{cert,key}.pem
  2. Set the owner of the directory and contents /etc/grid-security/xrd/ to xrootd:xrootd:
    [email protected] # chown -R xrootd:xrootd /etc/grid-security/xrd/


You must repeat the above steps whenever you renew your host certificate. If you automate certificate renewal, you should automate copying as well. For example, if you are using Certbot for Let's Encrypt, you should write a "deploy hook" as documented here.


You must also register this certificate with the CMS VOMS

Configuring Optional Features

Adjust disk utilization

To adjust the disk utilization of your cache, create or edit a file named /etc/xrootd/config.d/90-local.cfg and set the values of pfc.diskusage.

pfc.diskusage 0.90 0.95

The two values correspond to the low and high usage water marks, respectively. When usage goes above the high water mark, the XRootD service will delete cached files until usage goes below the low water mark.

Modify the storage access settings at a site

In order for CMSSW jobs to use the cache at your site you need to modify the storage.xml and create the following rules

# Portions of /store in xcache
  <lfn-to-pfn protocol="direct" destination-match=".*"
  <lfn-to-pfn protocol="direct" destination-match=".*"


If you are installing a multinode cache then instead of yourlocalcache:1094 url should be changed for yourcacheredirector:2040

Enable remote debugging

XRootD provides remote debugging via a read-only file system named digFS. This feature is disabled by default, but you may enable it if you need help troubleshooting your server.

To enable remote debugging, edit /etc/xrootd/digauth.cfg and specify the authorizations for reading digFS. An example of authorizations:

all allow gsi g=/glow h=*

This gives access to the config file, log files, core files, and process information to anyone from * in the /glow VOMS group.

See the XRootD manual for the full syntax.

Remote debugging should only be enabled for as long as you need assistance. As soon as your issue has been resolved, revert any changes you have made to /etc/xrootd/digauth.cfg.

Installing a Multinode Cache (optional)

Some sites would like to have a single logical cache composed of several nodes as shown below:

XRootD cluster

This can be achieved by following the next steps

Install an XCache redirector

This can be a simple lightweight virtual machine and will be the single point of contact from jobs to the caches.

  1. Install the redirector package

    [email protected] # yum install xcache-redirector --enablerepo=osg-development
  2. Create file named /etc/xrootd/config.d/04-local-redir.cfg with contents

    all.manager yourlocalredir:2041
  3. Start and enable the cmsd and xrootd proccess:

Software Service name Notes
XRootD [email protected] The cmsd daemon that interact with the different xrootd servers
XRootD [email protected] The xrootd daemon which performs authenticated data transfers

Configuring each of your cache nodes

  1. Create a config file in the nodes where you installed your caches /etc/xrootd/config.d/94-xrootd-manager.cfg with the following contents:

    :::file all.manager yourlocalredir:2041

  2. Start and enable the cmsd service:

Software Service name Notes
XRootD [email protected] The xrootd daemon which performs authenticated data transfers

Managing CMS XCache and associated services

These services must be managed by systemctl and may start additional services as dependencies. As a reminder, here are common service commands (all run as root) for EL7:

To... On EL7, run the command...
Start a service systemctl start <SERVICE-NAME>
Stop a service systemctl stop <SERVICE-NAME>
Enable a service to start on boot systemctl enable <SERVICE-NAME>
Disable a service from starting on boot systemctl disable <SERVICE-NAME>

CMS XCache services

Software Service name Notes
XRootD [email protected] The XRootD daemon, which performs the data transfers
XRootD (Optional) [email protected] The cmsd daemon that interact with the different xrootd servers
Fetch CRL fetch-crl-boot and fetch-crl-cron Required to authenticate monitoring services. See CA documentation for more info
xrootd-renew-proxy.service Renew a proxy for downloads to the cache
xrootd-renew-proxy.timer Trigger daily proxy renewal

XCache redirector services (Optional)

In the node where the cache redirector is installed these are the list of services:

Software Service name Notes
XRootD (Optional) [email protected] The xrootd daemon which performs authenticated data transfers
XRootD (Optional) [email protected] The xrootd daemon which performs authenticated data transfers

Validating the Cache

The cache server functions as a normal CMS XRootD server so first verify it with a personal CMS X.509 proxy:

=== VO cms extension information ===
VO        : cms
subject   : /DC=ch/DC=cern/OU=Organic Units/OU=Users/CN=efajardo/CN=722781/CN=Edgar Fajardo Hernandez
issuer    : /DC=ch/DC=cern/OU=computers/
attribute : /cms/Role=NULL/Capability=NULL
attribute : /cms/uscms/Role=NULL/Capability=NULL
timeleft  : 71:59:46
uri       :

Then test using xrdcp directly in your cache:

[email protected] $ xrdcp -vf -d 1 root://cache_host:1094//store/data/Run2017B/SingleElectron/MINIAOD/31Mar2018-v1/60000/9E0F8458-EA37-E811-93F1-008CFAC919F0.root /dev/null

Getting Help

To get assistance, please use the this page or contact directly.