The CernVM File System (CernVM-FS) is a read-only file system designed to deliver scientific software onto virtual machines and physical worker nodes in a fast, scalable, and reliable way. Files and file metadata are downloaded on demand and aggressively cached. For the distribution of files, CernVM-FS uses a standard HTTP [BernersLee96] [Fielding99] transport, which allows exploitation of a variety of web caches, including commercial content delivery networks. CernVM-FS ensures data authenticity and integrity over these possibly untrusted caches and connections. The CernVM-FS software comprises client-side software to mount “CernVM-FS repositories” (similar to AFS volumes) as well as a server-side toolkit to create such distributable CernVM-FS repositories.
The first implementation of CernVM-FS was based on grow-fs [Compostella10] [Thain05], which was originally provided as one of the private file system options available in Parrot. Ever since the design evolved and diverged, taking into account the works on HTTP- Fuse [Suzaki06] and content-delivery networks [Freedman03] [Nygren10] [Tolia03]. Its current implementation provides the following key features:
- Use of the the Fuse kernel module that comes with in-kernel caching of file data and file attributes
- Cache quota management
- Use of a content addressable storage format resulting in immutable files and automatic file de-duplication
- Possibility to split a directory hierarchy into sub catalogs at user-defined levels
- Automatic updates of file catalogs controlled by a time to live stored inside file catalogs
- Digitally signed repositories
- Transparent file compression/decompression and transparent file chunking
- Capability to work in offline mode providing that all required files are cached
- File system versioning
- File system hotpatching
- Dynamic expansion of environment variables embedded in symbolic links
- Support for extended attributes, such as file capabilities and SElinux attributes
- Automatic mirror server selection based on geographic proximity
- Automatic load-balancing of proxy servers
- Support for WPAD/PAC auto-configuration of proxy servers
- Efficient replication of repositories
- Possibility to use S3 compatible storage instead of a file system as repository storage
In contrast to general purpose network file systems such as nfs or afs, CernVM-FS is particularly crafted for fast and scalable software distribution. Running and compiling software is a use case general purpose distributed file systems are not optimized for. In contrast to virtual machine images or Docker images, software installed in CernVM-FS does not need to be further packaged. Instead it is distributed and versioned file-by-file. In order to create and update a CernVM-FS repository, a distinguished machine, the so-called Release Manager Machine, is used. On such a release manager machine, a CernVM-FS repository is mounted in read/write mode by means of a union file system [Wright04]. The union file system overlays the CernVM-FS read-only mount point by a writable scratch area. The CernVM-FS server tool kit merges changes written to the scratch area into the CernVM-FS repository. Merging and publishing changes can be triggered at user-defined points in time; it is an atomic operation. As such, a CernVM-FS repository is similar to a repository in the sense of a versioning system.
On the client, only data and metadata of the software releases that are actually used are downloaded and cached.