Implicit replication in a network file server

The design and implementation of a highly available network file server (HA-NFS) is reported. It is implemented on a network of workstations from the IBM RISC System/6000 family. HA-NFS servers preserve the semantics of the NFS protocol and can be used by existing NFS clients without modification. Therefore, existing application programs can benefit from highly availability without alteration. HA-NFS achieves storage reliability by (optionally) replicating files on different disks. However, all copies of the same file are controlled by a single server, reducing the cost of ensuring consistency. To achieve server reliability, each server is implicitly replicated by a backup that can access the server´s disks if the server fails. During normal operation, the backup monitors the liveness of the server but does not maintain information about the server´s internal state. Each server maintains a disk log that records state information normally kept in memory