US Patent 7072916 Instant snapshot

An improved method and apparatus for creating a snapshot of a file system. A record of which blocks are being used by a snapshot is included in the snapshot itself, allowing effectively instantaneous snapshot creation and deletion. The state of the active file system is described by a set of metafiles; in particular, a bitmap (henceforth the “active map”) describes which blocks are free and which are in use. The inode file describes which blocks are used by each file, including the metafiles. The inode file itself is described by a special root inode, also known as the “fsinfo block”. The system begins creating a new snapshot by making a copy of the root inode. This copy of the root inode becomes the root of the snapshot.