Experiences with two high availability designs [replication techniques]

The author compares two replication schemes designed to provide high availability in an efficient manner: HA-NFS and ARM. Both schemes use the primary copy method for replica control. Both schemes were designed with the goal of minimizing the overheads during failure-free operation. In a primary copy scheme these overheads primarily consist of updating the secondary replicas. The two schemes were designed for different applications; ARM for providing high availability in a Shared Nothing database system, HA-NFS for providing high availability in an NFS file server environment. They also differ in that the HA-NFS scheme uses dual-ported disks to provide high availability, the ARM scheme uses replication over a network. In spite of the seemingly major differences, the schemes have the same key conceptual idea viz. propagating updates asynchronously to remote replicas. In addition to this idea, HA-NFS uses an unusual hardware arrangement in the form of dual-ported disks to further lower the overhead of updating secondary replicas