Towards an efficient snapshot approach for virtual machines in clouds

High-availability of virtual machines is of significant importance for a cloud computing environment, because cloud services may be compromised due to the system maintenance, malicious attacks, and hardware and software failures. The virtual machine (VM) snapshot can effectively back up the state, disk data, and configuration of a machine at a specific time point. However, the existing VM snapshot methods suffer from performance issues such as long downtime and I/O performance degradation during a live snapshot. To address such issues, we proposed an efficient VM snapshot system, iROW (improved Redirect-on-Write), based on the qemu-kvm virtual block device driver. iROW employs the following techniques. (1) A bitmap-based light-weight index method is designed to reduce the query cost of the existing two-level index table structure compared with qcow2. (2) An index-free approach is used for VM state data to improve the performance of data saving and loading operations of VM state. (3) VM state data is separated from disk image data in a snapshot. (4) The free page detection (FPD) is designed using virtual machine introspection to identify and skip saving free pages in the guest OS during snapshotting, thus reducing the VM state snapshot creation time and the snapshot disk space usage. Our experimental results demonstrate that iROW is evidently advantageous over qcow2 in performance, in terms of the disk snapshot, the state snapshot and the disk I/O.