Re: [Qemu-devel] qemu-img convert cache mode for source

[Stefan Hajnoczi]

On Mon, Mar 03, 2014 at 01:20:21PM +0100, Peter Lieven wrote:
> On 03.03.2014 13:03, Stefan Hajnoczi wrote:
> >So what is the actual performance problem you are trying to solve and
> >what benchmark output are you getting when you compare with
> >FADV_DONTNEED against without FADV_DONTNEED?
> I found the performance to be identical. For the problem see below please.
> >
> >I think there's a danger that the discussion will go around in circles.
> >Please post the performance results that kicked off this whole effort
> >and let's focus on the data.  That way it's much easier to evaluate what
> >changes to QEMU are a win and which are not necessary.
> I found that under memory pressure situations the increasing buffers
> leads to vserver memory being swapped out. This caused trouble
> especially in overcommit scenarios (where all memory is backed by
> swap).

I think the general idea is qemu-img should not impact running guests,
even on a heavily loaded machine.  But again, this needs to be discussed
using concrete benchmarks with configurations and results posted to the
list.

Stefan