kvm-27 vs 28 I/O speed

[Dave Hansen <haveblue-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>]

I've noticed that some of my tests run *MUCH* slower in kvm-28 than in
27.  I'm sure that wall time is pretty wonky in the guests, but it is
much slower in real-world time as well.

Here's a little test to create a 32MB zeroed file with dd.  Here it is
from kvm-27 (this took ~5.5 seconds on my wristwatch):
33554432 bytes transferred in 0.052050 seconds (644657845 bytes/sec)
33554432 bytes transferred in 0.062933 seconds (533176451 bytes/sec)

Here's the same thing from kvm-28 (~80 seconds on my wristwatch):
33554432 bytes transferred in 38.607065 seconds (869127 bytes/sec)
33554432 bytes transferred in 22.274318 seconds (1506418 bytes/sec)

Same host kernel, same kvm kernel modules (from kvm-28) same guest
kernel, same command-line options, same disk image.

Any ideas what is going on?

-- Dave


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