CVMFS distributed file system?

Overview

Teaching: 40 min
Exercises: 0 min

Questions

• What is cvmfs

Objectives

• Understand the roles of the CVMFS.

CVMFS

What is CVMFS and why do we need it?
DUNE has a need to distribute precompiled code to many different computers that collaborators may use. Installed products are needed for four things:

1. Running programs interactively
2. Running programs on grid nodes
3. Linking programs to installed libraries
4. Inspection of source code and data files

Results must be reproducible, so identical code and associated files must be distributed everywhere. DUNE does not own any batch resources – we use CPU time on computers that participating institutions donate to the Open Science Grid. We are not allowed to install our software on these computers and must return them to their original state when our programs finish running so they are ready for the next job from another collaboration.

CVMFS is a perfect tool for distributing software and related files. It stands for CernVM File System (VM is Virtual Machine). Local caches are provided on each target computer, and files are accessed via the /cvmfs mount point. DUNE software is in the directory /cvmfs/dune.opensciencegrid.org, and LArSoft code is in /cvmfs/larsoft.opensciencegrid.org. These directories are auto-mounted and need to be visible when one executes ls /cvmfs for the first time. Some software is also in /cvmfs/fermilab.opensciencegrid.org.

CVMFS also provides a de-duplication feature. If a given file is the same in all 100 releases of dunetpc, it is only cached and transmitted once, not independently for every release. So it considerably decreases the size of code that has to be transferred.

When a file is accessed in /cvmfs, a daemon on the target computer wakes up and determines if the file is in the local cache, and delivers it if it is. If not, the daemon contacts the CVMFS repository server responsible for the directory, and fetches the file into local cache. In this sense, it works a lot like AFS. But it is a read-only filesystem on the target computers, and files must be published on special CVMFS publishing servers. Files may also be cached in a layer between the CVMFS host and the target node in a squid server, which helps facilities with many batch workers reduce the network load in fetching many copies of the same file, possibly over an international connection.

CVMFS also has a feature known as “Stashcache” or “xCache”. Files that are in /cvmfs/dune.osgstorage.org are not actually transmitted in their entirety, only pointers to them are, and then they are fetched from one of several regional cache servers or in the case of DUNE from Fermilab dCache directly. DUNE uses this to distribute photon library files, for instance.

CVMFS is by its nature read-all so code is readable by anyone in the world with a CVMFS client. CVMFS clients are available for download to desktops or laptops. Sensitive code can not be stored in CVMFS.

More information on CVMFS is available here

Exercise 6

• cd /cvmfs and do an ls at top level
• What do you see–do you see the four subdirectories (dune.opensciencegrid.org, larsoft.opensciencegrid.org, fermilab.opensciencegrid.org, and dune.osgstorage.org)
• cd dune.osgstorage.org/pnfs/fnal.gov/usr/dune/persistent/stash/PhotonPropagation/LibraryData

Useful links to bookmark

• CVMFS on DUNE wiki: Access files in CVMFS

Key Points

• CVMFS distributes software and related files without installing them on the target computer (using a VM, Virtual Machine).