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

[Avi Kivity]

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?

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