Name space consistency in the Pangaea wide - area file system HPL

Pangaea is a wide-area file system that enables ad-hoc collaboration in multi-national corporations or in distributed groups of users. This paper describes Pangaea’s approach for keeping the file-system’s name space consistent and proves its correctness. Maintaining the name space is a simple matter in traditional file systems that store the entire volume in a single node. It is not so in Pangaea, because of the two key techniques it employs to improve performance and availability in a wide area— pervasive replication that lets each file be replicated on its own set of nodes on demand from users, and optimistic replication that lets updates be issued on any replicas at any time. A naive implementation may leave some files unreachable in the name space or some directory entries pointing to non-existent files. To detect conflicting updates and inform all affected replicas about the resolution outcome reliably, Pangaea embeds, in each file, a data structure called backpointer that authoritatively defines the file’s location in the file-system’s name space. Conflicting directory operations are detected by a replica of the (child) file as a discrepancy in the value of the backpointer. The replica can then unilaterally resolve conflicts and disseminate the conflict resolution outcome to the the parent directories.