[Qemu-devel] [RFC] Propose the Fast Virtual Disk (FVD) image format that outperforms QCOW2 by 249%

[Chunqiang Tang]

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.

Best Regards,
Chunqiang Tang 

Homepage: http://www.research.ibm.com/people/c/ctang