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

[Avi Kivity <avi@redhat.com>]

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?

-- 
error compiling committee.c: too many arguments to function