iROW: An Efficient Live Snapshot System for Virtual Machine Disk

The high-availiablity of mission-critical data and services hosted in a virtual machine (VM) is one of the top concerns in a cloud computing environment. The live disk snapshot is an emerging technology to save the whole state and the data of a VM at a specific point of time, and be used for quick disaster recovery. However, the existing VM disk snapshot systems suffer from long operation time and I/O performance degradation problems during snapshots creating and managing, and thereby affecting the performance of the VM and its services. To address such issues, we designed an efficient VM disk snapshot system, named iROW (improved Redirect-on-Write). In iROW, a bitmap based light-weight index scheme is adopted to replace the existing multi-level index tree structure to reduce query cost. Additionally, through a combination of Redirect-on-Write (ROW) and Copy-on-Demand (COD) schema to avoid extra copy operation on the first write after snapshot with Copy-on-Write (COW) schema, and the file fragmentation problem caused by ROW snapshot after long-term using. Finally, iROW gives a unified disk space allocation function by the host machine´s file system. We have implemented iROW in qemu-kvm 0.12.5 and conducted some experiments. The implementation of iROW completely obey the interfaces of the block device driver in QEMU, so it is transparent to the upper system or applications and original disk image formats can be also supported. The experimental results show that iROW has obvious performance advantages in snapshot creating and management operations. Compared with the existing qcow2 disk image in KVM, when the VM disk size is 50GB, and the cluster size is 64KB (the default cluster size of qcow2), the snapshot creation and rollback time is only about 6% and 3% of original qcow2´s. With the increasing of the VM disk size, iROW has more performance advantages on snapshot creation and rollback operations. In addition, the I/O performance of iROW is better than qcow2. Whe- the cluster size is 64 KB, typically the iROW´s performance loss is 10% less than qcow2´s, and its first write performance after snapshot creation is about 250% of qcow2´s.