Prism: providing flexible and fast filesystem cloning service for virtual servers
Proceedings of the 9th ACM/IFIP/USENIX International Conference on Middleware
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Virtual machine technology is being widely used for many applications such as server consolidation, software testing, and fault isolation. While it provides tremendous benefits to users, it also imposes higher requirements on VM storage manageability, flexibility, and security. Unfortunately, the main form of storage for conventional virtual machines, virtual disks, have serious limitations with respect to flexibility and security. This dissertation proposes a virtualization aware file system (VAFS) named Prism, which works as an alternative to virtual disks, but helps improve manageability, flexibility, and security of virtual machines. Prism extends a conventional distributed filesystem, NFSv3, with new features such as cloning, sharing, and isolation. It presents each VM with a dedicated filesystem that is semantically isolated from filesystems in other VMs. Guest applications can read and write files on the Prism filesystem without requiring any modification. As a central storage, Prism can map identical files in different VMs to one file on disk, substantially reducing storage and disk I/O overhead. If one attempts to write a shared file, Prism performs copy-on-write to preserve isolation between VMs. The thesis describes the Prism file mapping and copy-on-write mechanisms. Prism offers a cloning primitive that can selectively clone one or more VMs' filesystems to compose a new VM's filesystem. The Prism cloning primitive is semantically similar to a copy operation in a conventional filesystem, except much faster. From an end-user perspective, cloning is completed instantaneously, irrespective of the size of the cloned filesystem. The Prism cloning mechanism uses disk space very efficiently. The parent and cloned filesystem share data of unchanged files, which usually occupy a large portion of files. Prism only consumes additional space to store changes between the parent and its clone. In addition, this dissertation proposes two techniques, Virtual Remote Procedure Call (VRPC) and Inter-VM Metadata Sharing , to improve the Prism performance. The two techniques use shared memory to substantially reduce the overhead of Prism communication between VMs on a same physical computer. Experiments show that the two techniques can make Prism performance close to a local filesystem running on virtual disks for normal workloads.