Over one year has passed since my last virtual machine hypervisor comparison so, in the last week, I was preparing an article showing a face to face comparison between RHEL 6 KVM technologies versus Oracle VirtualBox 4.0 product. I spent several days creating some nice, automated script to evaluate these two products under different point of views, and I was quite confident that the benchmark session would be completed without too much trouble. So, I installed Red Hat Enterprise Linux 6 (license courtesy of Red Hat Inc. - thank you guys!) on my workstation and I begin the virtual images installation.

However, the unexpected happened: using KVM, a Windows Server 2008 R2 Foundation installation took almost 3 hours, while normally it should be completed in about 30-45 minutes. Similarly, the installation of the base system anticipating the “real” Debian 6.0 installation took over 5 minutes, when normally it can be completed in about 1 minute. In short: the KVM virtual machines were affected by awfully slow disk I/O subsystem. In previous tests, I saw that KVM I/O subsystem was a bit slower, but not by so much; clearly, something was impairing my KVM I/O speed. I tried different combination of virtualized disk controllers (IDE or VirtIO) and cache settings, but without success. I also changed my physical disk filesystem to EXT3, to avoid any possible, hypothetical EXT4 speed regression, but again with no results: the KVM slow I/O speed problem remained.

I needed a solution – and a real one: with such awfully slow I/O, the KVM guests were virtually unusable. After some wasted hours, I decided to run some targeted, systematic tests regarding VM image formats, disk controllers, cache settings and preallocation policy. Now that I find the solution and I run my KVM guest at full speed, I am very happy – and I would like to share my results with you.