Meta-data snapshotting: a simple mechanism for file system consistency

File system consistency frequently involves a choice between raw performance and integrity guarantees. A few software-based solutions for this problem have appeared and are currently being used on some commercial operating systems; these include log-structured file systems, journaling file systems, and soft updates. In this paper, we propose meta-data snapshotting as a low-cost, scalable, and simple mechanism that provides file system integrity. It allows the safe use of write-back caching by making successive snapshots of the meta-data using copy-on-write, and atomically committing the snapshot to stable storage without interrupting file system availability. In the presence of system failures, no file system checker or any other operation is necessary to mount the file system, therefore it greatly improves system availability. This paper describes meta-data snapshotting, and its incorporation into a file system available for the Linux and K42 operating systems. We show that meta-data snapshotting has low overhead: for a microbenchmark, and two macrobenchmarks, the measured overhead is of at most 4%, when compared to a completely asynchronous file system, with no consistency guarantees. Our experiments also show that it induces less overhead then a write-ahead journaling file system, and it scales much better when the number of clients and file system operations grows.Furthermore, this new technique can be easily extended to provide file system snapshotting (versioning) and transaction support for a collection of selected files or directories.