Towards Virtual Machine Image Management for Persistent Memory

Persistent memory's (PM) byte-addressability and high capacity will also make it emerging for virtualized environment. Modern virtual machine monitors virtualize PM using either I/O virtualization or memory virtualization. However, I/O virtualization will sacrifice PM's byte-addressability, and memory virtualization does not get the chance of PM image management. In this paper, we enhance QEMU's memory virtualization mechanism. The enhanced system can achieve both PM's byte-addressability inside virtual machines and PM image management outside the virtual machines. We also design pcow, a virtual machine image format for PM, which is compatible with our enhanced memory virtualization and supports storage virtualization features including thin-provision, base image and snapshot. Address translation is performed with the help of Extended Page Table (EPT), thus much faster than image formats implemented in I/O virtualization. We also optimize pcow considering PM's characteristics. The evaluation demonstrates that our scheme boosts the overall performance by up to 50x compared with qcow2, an image format implemented in I/O virtualization, and brings almost no performance overhead compared with the native memory virtualization.