Efficient Replication of Large Data Objects

We present a new distributed data replication algorithm tailored especially for large-scale read/write data objects such as files. The algorithm guarantees atomic data consistency, while incurring low latency costs. The key idea of the algorithm is to maintain copies of the data objects separately from information about the locations of up-to-date copies. Because it performs most of its work using only the location information, our algorithm needs to access only a few copies of the actual data; specifically, only one copy during a read and only f+1 copies during a write, where f is an assumed upper bound on the number of copies that can fail. These bounds are optimal. The algorithm works in an asynchronous message-passing environment. It does not use additional mechanisms such as group communication or distributed locking. It is suitable for implementation in WANs as well as LANs. We also present two lower bounds on the costs of data replication. The first lower bound is on the number of low-level writes required during a read operation on the data. The second bound is on the minimum space complexity of a class of efficient replication algorithms. These lower bounds suggest that some of the techniques used in our algorithm are necessary. They are also of independent interest.