CernVM-FS Terminology¶
An overview of terms used in the context of CernVM-FS, in alphabetical order.
Catalog¶
A catalog of a CernVM-FS repository is a table that lists files and directories along with the corresponding metadata (sizes, timestamps, etc.).
Catalogs can be nested: subtrees of the repository may have their own catalog.
For more information on the catalog concept, see the CernVM-FS documentation.
CernVM¶
CernVM is a virtual machine image based on CentOS combined with a custom, virtualization-friendly Linux kernel, and which includes the CernVM-FS client software.
It is used for the CERN Large Hadron Collider (LHC) experiment, and was created to remove a need for the installation of the experiment software and to minimize the number of platforms (compiler-OS combinations) on which experiment software needs to be supported and tested.
While originally developed in conjunction, the CernVM File System today is a product that is completely independent from the CernVM virtual appliance.
For more information on CernVM, see the website and documentation.
CernVM-FS¶
(see What is CernVM-FS?)
Client¶
A client in the context of CernVM-FS is a computer system on which a CernVM-FS repository
is being accessed, on which it will be presented as a POSIX
read-only file system in a subdirectory of /cvmfs.
Proxy¶
A proxy, also referred to as squid proxy, is a forward caching proxy server which acts as an intermediary between a CernVM-FS client and the Stratum-1 replica servers.
It is used to improve the latency observed when accessing the contents of a repository, and to reduce the load on the Stratum-1 replica servers.
A commonly used proxy is Squid.
For more information on proxies, see the CernVM-FS documentation.
Publishing¶
Publishing is the process of adding more files to a CernVM-FS repository, which is done via a transaction mechanism, and is on possible on the Stratum-0 server, via a publisher, or via a repository gateway.
The workflow of publishing content is covered in detail in the CernVM-FS documentation.
Repository¶
A CernVM-FS repository is where the files and directories that you want to distribute via CernVM-FS are stored, which usually correspond to a collection of software installations.
It is a form of content-addressable storage (CAS),
and is the single source of (new) data for the file system being presented as a subdirectory of /cvmfs
on client systems that mount the repository.
Note
A CernVM-FS repository includes software installations, not software packages like RPMs.
Software installations¶
An important distinction for a CernVM-FS repository compared to the more traditional notion of a software repository is that a CernVM-FS repository provides access to the individual files that collectively form a particular software installation, as opposed to housing a set of software packages like RPMs, each of which being a collection of files for a particular software installation that are packed together in a single package to distribute as a whole.
Note
This is an important distinction, since CernVM-FS enables only downloading the specific files that are required to perform a particular task with a software installation, which often is a small subset of all files that are part of that software installation.
Stratum 0 server¶
A Stratum 0 server, often simply referred to a Stratum 0 (Stratum Zero), is the central server for one or more CernVM-FS repositories.
It is the single source of (new) data, since it hosts the master copy of the repository contents.
Adding or updating files in a CernVM-FS repository (publishing) can only be done on the Stratum 0 server,
either directly via the cvmfs_server publish command, or indirectory via a publisher server.
For more information, see the CernVM-FS documentation.
Stratum 1 replica server¶
A Stratum 1 replica server, often simply referred to a Stratum 1 (Stratum One), is a standard web server that acts as a mirror server for one or more CernVM-FS repositories.
It holds a complete copy of the data for each CernVM-FS repository it serves, and automatically synchronises with the Stratum 0.
There is typically a network of several Stratum 1 servers for a CernVM-FS repository, which are geographically distributed.
Clients can be configured to automatically connect to the closest Stratum 1 server by using the CernVM-FS GeoAPI.
For more information, see the CernVM-FS documentation.