An Implementation of Semi-synchronous Remote Mirroring System for SANs

摘要

Remote mirroring is often used as part of disaster recovery solutions. Synchronous remote mirroring incurs steep costs in both write latency and network bandwidth to support the mirroring, while asynchronous mirroring does not ensure the consistency of remote data. In this paper, we designed and implemented a storage-based semi-synchronous remote mirroring system for SANs. By using a log policy for the active remote write commands, this approach allows a limited number of write I/O operations to proceed before waiting for acknowledgment of receipt from the remote site, which significantly reduces write latency. This implementation also provides a consistent copy in a remote site to meet the demand for disaster recovery, because the order of commands is guaranteed. Furthermore, it can be applied to the working condition of long distance mirroring with high network latency. The testing results show that with the same request size and network latency, our semi-synchronous remote mirroring reduces average write command response time by 14-20% compared with synchronous remote mirroring.