This post is about a nice discovery I made when I upgraded my XEN dom0 kernel to the latest one (2.6.32-).
I suddenly noticed that the I/O performance was highly dropping down when moving few small files (~350MB) from one partition to another one of the same hard-disk.
I worried myself so I began investigating about the issue and, eventually, I posted on xen-users mailing list .
After few days I’ve been told that the problem was probably given by the I/O scheduler on the guest machine.
The solution is to disable domU scheduler and leave all the work to the dom0 scheduler. And it, actually, did the trick for me.
In order to do that, just add ‘elevator=noop’ to the kernel’s bootflag of your guest machine and check if the system becomes more reactive.
If you are interested into the whole dynamic or you’re simply willing to understand better the reasons why this happens, here’s the thread on XEN mailing list.
If you’ll find a possible explanation for such a slow-down.
Just to be curious: drop me a comment about if something changes or not on your systems.