A Fan-Out Snapshotting File System

Snapshots allow users to modify the file system, while maintaining the ability to view the file system precisely as it was when a snapshot was taken. Snapshots are useful because they provide versioning, and allow consistent access to an entire tree (e.g., so that backups can be taken on a live system).

We will build a fan-out stackable snapshotting file system that supports an arbitrarily large number of snapshots, using Unionfs to provide the core fan-out infrastructure. When a snapshot is taken, an empty high priority branch will be inserted into the fan. Any modifications are made within the new branch, and the original data is preserved in the the previous branches.

There are three methods of accessing a snapshot. A user may use a separate name space (e.g., /snapshots/03132003 for March 13, 2003). A special operator may be used to access the previous snapshot (e.g., ... will go to the previous snapshot of that directory). Finally, We will update the VFS to allow one path to have multiple per-entity trees. The user can run a new utility set-snap that informs Snapshotfs which view the user or process is using.

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