The CernVM-File System (CernVM-FS) provides a scalable, reliable and low- maintenance software distribution service. It was developed to assist High Energy Physics (HEP) collaborations to deploy software on the worldwide- distributed computing infrastructure used to run data processing applications. CernVM-FS is implemented as a POSIX read-only file system in user space (a FUSE module). Files and directories are hosted on standard web servers and mounted in the universal namespace /cvmfs. Internally, CernVM-FS uses content- addressable storage and Merkle trees in order to maintain file data and meta-data. CernVM-FS uses outgoing HTTP connections only, thereby it avoids most of the firewall issues of other network file systems. It transfers data and meta-data on demand and verifies data integrity by cryptographic hashes.

By means of aggressive caching and reduction of latency, CernVM-FS focuses specifically on the software use case. Software usually comprises many small files that are frequently opened and read as a whole. Furthermore, the software use case includes frequent look-ups for files in multiple directories when search paths are examined.

Content is published into /cvmfs by means of dedicated "release manager machines". The release manager machines provide a writeable CernVM-FS instance by means of a union file system (aufs or overlayfs) on top of the read-only client. When publishing, the CernVM-FS server tools process new and modified data from the union file system's writable branch and transform the data into the CernVM-FS storage format.

CernVM-FS is actively used by small and large scientific collaborations. In many cases, it replaces package managers and shared software areas on cluster file systems as means to distribute the software used to process experiment data.