A Log Structured File System with Snapshots

A log structured file system (LFS) [1] writes all the file system data sequentially in a log-like structure. A log consists of a series of segments where each segment contains both data and inode blocks. Traditional file systems like ext2 usually write inode blocks at a fixed place on the disk, causing overhead due to disk seeks. A log structured file system gathers a segment worth of data in memory and appends the segment at the end of the log. This dramatically improves the write performance while maintaining the same read performance. The sequential nature of the log also helps in crash recovery as less checkpointing information need to be stored. As the file system grows and files are deleted, holes are created in the file system. A cleaner is required to fill the holes and compact the file system allowing large extents of free blocks to be found. The novel aspect in our work is the addition of snapshotting capability to log-structured file systems. Currently, no Linux file system offers this capability. The primary objective of this work is to create a log-structured file system for Linux that supports snapshots. A snapshot is a copy of the files taken at a particular time. This is very similar to backup of a file system at a particular time except that it is maintained within the same file system without wasting any space. We believe that LFS is the ideal file system for maintaining snapshots, because its design renders naturally to maintain snapshots.