Re: [Qemu-devel] [PATCH] qcow2: Order concurrent AIO requests on the same unallocated cluster

[Gleb Natapov]

On Tue, Sep 01, 2009 at 02:24:46PM +0300, Avi Kivity wrote:
> On 09/01/2009 01:50 PM, Kevin Wolf wrote:
> >
> >>Can't this cause an even/odd pattern where all even-numbered requests
> >>run first, then all the odd-numbered requests?
> >>
> >>(0 goes to disk, 1 depends on it, 2 depends on 1, which isn't allocating
> >>now, so it goes to disk, 3 depends on 2, ...)
> >I guess it can happen in theory, not sure if it matters in practice.
> 
> We should check then.
> 
> >  You
> >are worried about image fragmentation? I think we could do something
> >about it with a cleverer cluster allocation.
> 
> Not only image fragmentation - the odd requests will require RMW.
> 
> >However, I don't think it's an argument against this patch. What
> >currently happens isn't much better: Allocate n clusters, free n-1.
> >Almost as good in producing fragmentation.
> 
> The patch introduces complexity so it makes working towards a
> non-fragmenting solution harder.  I'm not saying it could be
> simplified, it's a side effect of using a state machine design.
> 
> >>Do you have performance numbers?
> >No really detailed numbers. Installation time for RHEL on qcow2/virtio
> >went down from 34 min to 19 min, though.
> 
> That's very impressive.  cache=none or cache=wt?
> 
And how it compares with raw?

--
                        Gleb.