Efficient Logging and Replication Techniques for Comprehensive Data Protection

Mariner is an iSCSI-based storage system that is designed to provide comprehensive data protection on commodity ATA disk and gigabit Ethernet technologies while offering the same performance as those without any such protection. In particular, Mariner supports continuous data protection (CDP) that allows every disk update within a time window to be undoable, and local/remote mirroring to guard data against machine/site failures. To minimize the performance overhead associated with CDP, Mariner employs a modified track-based logging technique that unifies the long-term logging required for CDP and short-term logging for low-latency disk writes. This new logging technique strikes an optimal balance among log space utilization, disk write latency, and ease of historical data access. To reduce the performance penalty of physical data replication used in local/remote mirroring, Mariner features a modified two-phase commit protocol that in turn is built on top of a novel transparent reliable multicast (TRM) mechanism specifically designed for Ethernet-based storage area networks. Without flooding the network, TRM is able to keep the network traffic load of reliable N-way replication roughly at the same level as the no-replication case, regardless of the value of N. Empirical performance measurements on the first Mariner prototype, which is built from gigabit Ethernet and ATA disks, shows that the average end-to-end latency for a 4KByte iSCSI write is under 1.2 msec when data logging and replication are both turned on.