Almost one year ago, I checked how different cache settings affected KVM storage subsystem performance. Results were very clear: to obtain good I/O speed, you had to use the write-back or none cache policies, avoiding the write-through one. However, as the write-back policy intrinsic comported some data-loss risk, the safer bullet was to not use any host-based cache (the “nocache” KVM option).
But what is the situation now? With the newly released RHEL 6.2 point release, I am going to check if something has changed. But let me first recap the whole KVM caching question.