Several existing distributed file systems provide reliability by server replication. The servers implement protocols that ensure the consistency and coherence of the replicas. There are several advantages to this approach: fast recovery from failure, flexibility, and ability to distribute file reads among replicas, and resistance to site disaster. These advantages come at the expense of an increase in network and CPU loads.
An alternative approach is to use dual-ported disks accessible to a server and a backup. The server stores information about its volatile mate on a disk log. If the server fails, the backup will reconstruct the server’s lost state using the log and will impersonate the server in addition to maintaining its own identity.
This paper compares the two approaches by addressing the issues of availability, overhead during normal operation, flexibility and cost of reintegrating failed servers after recovery.