File Versioning for Block-Level Continuous Data Protection

Block-level continuous data protection (CDP) logs every disk block update so that disk updates within a time window are undoable. Standard file servers and DBMS servers can enjoy the data protection service offered by block-level CDP without any modification. Unfortunately, no existing block-level CDP systems can provide users a file versioning view on top of the block versions they maintain. As a result, the data they maintain cannot be used as an extension to the on-line system with which users routinely interact. This paper describes a name-based user-level file versioning system called UVFS that is designed to reconstruct file versions from disk block versions maintained by a block-level CDP. UVFS reconstructs file versions by following the last modified time of files and directories, a common file metadata supported by almost all modern file systems, and therefore does not require any modification to the host file system that a block-level CDP system protects. In addition, UVFS incorporates a file system-specific incremental consistency check mechanism to quickly convert an arbitrary point-in-time block-level snapshot to a file system-consistent one. Performance measurements taken from a fully operational UVFS prototype show that the average end-to-end elapsed time required to discover a file version is under 50 msec from the perspective of an NFS client serviced by an NFS server backed by a block-level CDP system.