You can add cache=none to the options when defining a disk. This has been tested to eliminate the performance hit of caching and increase IO performance of guests.
http://permalink.gmane.org/gmane.comp.emulators.kvm.devel/48471
http://www.linux-kvm.org/page/Tuning_KVM explains the situation a bit more.
http://permalink.gmane.org/gmane.comp.emulators.kvm.devel/48471
http://www.linux-kvm.org/page/Tuning_KVM explains the situation a bit more.
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