The expandable network disk

Abstract

This thesis presents a virtual disk cluster called END, the Expandable Network Disk. END aggregates storage on a cluster of servers into a single virtual disk. END's main goals are to offer good performance during normal operation, and efficiently handle changes in the cluster membership. END achieves these goals using a two-layer design, in which storage "bricks," servers that consist of CPU, memory, and hard disks, hold two kinds of information. The lower layer stores replicated immutable chunks of data, each indexed by a unique key. The upper layer maps each block address to the key of its current data chunk; each mapping is held on two bricks using primary-backup replication. This separation allows END flexibility in where it stores chunks and thus efficiency: it writes new chunks to bricks chosen for speed; it moves only address mappings (not data) when bricks fail and recover, which results in fast recovery; it fully replicates new writes during temporary brick failures; and it uses chunks on a recovered brick without risk of staleness. The END prototype's write throughput on a cluster of 16 PC-based bricks is 150 megabytes/s with 2x replication. END continues after a single brick failure, re-incorporates a rebooting brick, and expands to include a new brick after a few seconds of reduced performance during each change. The results show that END's two-layer design maintains good performance, resumes operation quickly after changes in the cluster, and maintains full replication of new writes even during a brick failure.