Versioning file system
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A versioning file system is a file system which provides for the concurrent existence of several versions of a file. In their most common form, they keep a configurable number of older copies of a file around, periodically creating a new copy of the file with a new version number. These new file instances are normally created every time the file is updated, although some versioning file systems limit the number of changes per minute or per hour to avoid storing large numbers of trivial changes.
Conceptually, a versioning file system is similar to a periodic backup, with several key differences. First, backups normally go to separate media; versioning file systems write to the same hard drive (and normally the same folder, directory, or local partition). Second, backups are normally triggered on a timed basis; versioning occurs when the file changes. Third, backups are usually system-wide or partition-wide; versioning occurs independently on a file-by-file basis.
Versioning file systems provide some of the features of revision control systems. However, unlike most revision control systems, they are transparent.
Versioning file systems should not be confused with journaling file systems.
[edit] Implementations
It was first introduced by the TENEX operating system.
A simple but powerful example of a file versioning system is built into the VMS operating system from Digital Equipment Corporation (DEC). In essence, whenever an application opens a file for writing, the file system automatically creates a new instance of the file, with a version number appended to the name. Version numbers start at 1 and count upward automatically as new instances of a file are created. When an application opens a file for reading, it can either specify the exact file name including version number, or just the file name without the version number, in which case the most recent instance of the file is opened. The file system automatically deletes sufficiently old versions of a file when a new instance is created; the maximum number of files retained is configurable, with an upper limit of 32,767.
See Wayback or CopyFS for examples of a versioning file system for Linux.
Network Appliance's storage solutions implement a file system called WAFL, which uses snapshot technology to keep different versions of all files in a volume around. Another example is ext3cow. While these are not strictly versioning file systems, they are related.
There is also the tool pdumpfs, which makes it possible to copy a whole directory to another location by using hardlinks. Used regularly, this can produce much the same effect as the tools mentioned in the previous paragraph. (Though they are still not really versioning file systems.)
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