On Improving the Availability of Replicated Files

To improve the availability and reliability of files the data are often replicated at several sites. A scheme must then be chosen to maintain the consistency of the file contents in the presence of site failures. The most commonly used scheme is voting. Voting is popular because it is simple and robust: voting schemes do not depend on any sophisticated message passing scheme and are unaffected by network partitions. When network partitions cannot occur, better availabilities and reliabilities can be achieved with the available copy scheme. This scheme is somewhat more complex than voting as the recovery algorithm invoked after a failure of all sites has to know which site failed last. We present in this paper a new method aimed at finding this site. It consists of recording those sites which received the most recent update; this information can then be used to determine which site holds the most recent version of the file upon site recovery. Our approach does not require any monitoring of site failures and so has a much lower overhead than other methods. We also derive, under standard Markovian assumptions, closed-form expressions for the availability of replicated files managed by voting, available copy and a näıve scheme that does not keep track of the last copy to fail.