Dear QEMU Community Members,
Happy new year! We would like to contribute a new year gift to the
community.
As the community considers the next-generation image formats for QEMU,
hopefully we really challenge ourselves hard enough to find the right
solution for the long term, rather than just a convenient solution for the
short term, because an image format has long-term impacts and is hard to
change once released. In this spirit, we would like to argue that QCOW2
and QED’s use of a two-level lookup table as the basis for implementing
all features is a fundamental obstacle for achieving high performance.
Accordingly, we advocate the newly developed Fast Virtual Disk (FVD) image
format for adoption in the QEMU mainline. FVD achieves the performance of
a RAW image running on a raw partition, while providing the rich features
of compact image, copy-on-write, copy-on-read, and adaptive prefetching.
FVD is extensible and can accommodate additional features. Experiments
show that the throughput of FVD is 249% higher than that of QCOW2 when
using the PostMark benchmark to create files.
FVD came out of the work done at IBM T.J. Watson Research Center, when
studying virtual disk related issues during the development of the IBM
Cloud (http://www.ibm.com/services/us/igs/cloud-development/). At IBM
internally, FVD (a.k.a. ODS) has been widely demonstrated since June 2010.
Recently, the FVD technical papers were completed and the source code was
cleared for external release. Now we finally can share FVD with the
community, and seek your valuable feedback and contributions. All related
information is available at
https://researcher.ibm.com/researcher/view_project.php?id=1852 , including
a high-level overview of FVD, the source code, and the technical papers.
The FVD patch also includes a fully automated testing framework that
exercises QEMU block device drivers under stress load and extreme race
conditions. Currently (as of January 2011), QCOW2 cannot pass the
automated test. The symptom is that QCOW2 attempts to read beyond the end
of the base image. QCOW2 experts please take a look at this "potential"
bug.