A Survey on Copy-on-Write File Systems

Crash consistency is crucial for ensuring the correctness of a storage system. One of the three classic file system consistency techniques is shadow paging (i.e., CoW). In CoW file system, instead of in-place modifying data on disk, the file system writes updates to newly-allocated blocks. It copies over old blocks and newly-allocated ones if it is necessary. After the updates have been confirmed as completed, the old ones can be released. CoW file systems, such as ZFS, BTRFS, BCacheFS, have still attracted attention in usage and development for years. This study firstly provides an overview of a CoW filesystem. Second, the comparison of CoW file systems in terms of provided features is provided. Besides, we compare CoW file systems to popular file systems applying the two other consistency techniques (i.e., journaling and log-structuring file systems) in terms of provided features.