File System Design for an NFS File Server Appliance

Network Appliance recently began shipping a new kind of network server called an NFS file server appliance, which is a dedicated server whose sole function is to provide NFS file service. The file system requirements for an NFS appliance are different from those for a general-purpose UNIX system, both because an NFS appliance must be optimized for network file access and because an appliance must be easy to use. This paper describes WAFL (Write Anywhere File Layout), which is a file system designed specifically to work in an NFS appliance. The primary focus is on the algorithms and data structures that WAFL uses to implement Snapshots , which are read-only clones of the active file system. WAFL uses a copy-on-write technique to minimize the disk space that Snapshots consume. This paper also describes how WAFL uses Snapshots to eliminate the need for file system consistency checking after an unclean shutdown.