Virtual Disk Image Reclamation for Software Updates in Virtual Machine Environments

Virtual disks (VD) are the main form of storage in today's virtual machine (VM) environments for they have many attractive features, such as encapsulation, mobility, isolation, etc. A typical virtual disk image usually has a large size of at least several gigabytes, a disadvantage that may limit its usage in many situations, such as fast software deployment in large-scale virtual machine environments, fast VM migration in local or distributed environments, etc. The COW (Copy-on-Write) virtual block device supported by many virtual machine monitors can split the traditional one-piece large-sized VD image into multiple smaller-sized VD images (COW disks). However, frequent or long-term software updates may result in large numbers of COW disks. Moreover, the accumulated large numbers of old-version COW disks (OCD) installed with old-version software may cause serious disk space wastage. In this paper, we propose two VD image reclamation approaches RPD and RMD to reclaim the OCDs so as to free the disk space taken by the old-version software and to maintain the number of COW disks in a specific range to ease the VD image management. Based on QEMU and Linux system, we have developed tools to implement the two approaches. Experiments show that the two VD image reclamation approaches can effectively reclaim the OCDs so as to be able to support frequent or long-term software updates in virtual machine environments.