PersiFS: a versioned file system with an efficient representation

The availability of previous file versions is invaluable for recovering from file corruption or user errors such as accidental deletions. Versioned file systems address this need by retaining earlier versions of changed files. Many existing file systems, such as Plan 9, WAFL, AFS, and others, use a snap-shotting approach: they record and archive the state of the file system at periodic intervals. However, this fails to capture modifications that are made between snapshots. Our system, PersiFS, is continuously versioned, meaning that it stores every modification, and thus allows access to the file system state as it appeared at any specified time. To make this feasible, we use a number of efficient data structures to optimize both access time and disk space.