ConSnap: Taking continuous snapshots for running state protection of virtual machines

Abstract

The reliability of data and services hosted in a virtual machine (VM) is a top concern in cloud computing environment. Continuous snapshots reduces the data loss in case of failures, and thus is prevailing for providing protection for long-running systems. However, existing methods suffer from long VM downtime, long snapshot interval and significant performance overhead. In this paper, we present ConSnap, a system designed to enable taking fine-grained continuous snapshots of virtual machines without compromising VM performance. First, ConSnap adopts the COW (copy-on-write) manner to save the memory pages in a lazy way, and thus decrease the snapshot interval to dozens of milliseconds. Second, we only save the incremental memory pages on the basis of the last snapshot in each epoch to reduce the snapshot duration, and thus mitigate VM performance loss. Third, we propose a multi-granularity space reclamation strategy, which merges the unused snapshot files to achieve storage space saving, as well as fast recovery. We have implemented ConSnap on QEMU/KVM and conducted several experiments to verify its effectiveness. Compared with the stop-and-copy based incremental snapshots, ConSnap reduces the performance loss by 71.1% ~ 10.2% under Compilation workload, and 14.5% ~ 4.7% for the Ftp workload, when the interval varies from 1s to 60s.