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

[Avi Kivity]

On 09/01/2009 02:43 PM, Kevin Wolf wrote:
> Avi Kivity schrieb:
>    
> > On 09/01/2009 01:50 PM, Kevin Wolf wrote:
> >      
> > >    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.
> >      
> How that?
>
> The case you're thinking of is that the first and third request are
> already completed and then the second one starts, right? Assuming that
> request 2 involves some sectors in the last cluster of 1 and the first
> one of 3.
>
> Then request 2 is written in three phases: The first one overwrites the
> last sectors of requests 1 (cluster already allocated =>  no RMW). The
> second one writes to unallocated, cluster-aligned space (writing
> complete clusters =>  no RMW). The third one overwrites the first sectors
> of request 3 (cluster already allocated =>  no RMW).
>    

Ah, ok.  No RMW, but a three-way split as well as fragmentation.

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