Re: [Qemu-devel] [PATCH] qcow2: Metadata preallocation

[Avi Kivity]

On 08/17/2009 10:11 AM, Kevin Wolf wrote:
> Avi Kivity schrieb:
>    
> > On 08/14/2009 06:00 PM, Kevin Wolf wrote:
> >      
> > > This introduces a qemu-img create option for qcow2 which allows the metadata to
> > > be preallocated, i.e. clusters are reserved in the refcount table and L1/L2
> > > tables, but no data is written to them. Metadata is quite small, so this
> > > happens in almost no time.
> > >
> > > Especially with qcow2 on virtio this helps to gain a bit of performance during
> > > the initial writes. However, as soon as create a snapshot, we're back to the
> > > normal slow speed, obviously. So this isn't the real fix, but kind of a cheat
> > > while we're still having trouble with qcow2 on virtio.
> > >
> > > Note that the option is disabled by default and needs to be specified
> > > explicitly using qemu-img create -f qcow2 -o preallocation=metadata.
> > >
> > >
> > >        
> > Can't say I'm thrilled with this.  I'd prefer coalescing metadata
> > updates on parallel writes.  I don't object to this though.
> >      
> Even with improved concurrent cluster allocation, you might profit from
> metadata preallocation by having less fragmented qcow2 images which
> avoids splitting up requests. Not sure if this is relevant in practice
> though.
>    

What I meant was that I prefer changes that improve performance 
throughout the lifetime of the image rather than the initial writes, 
especially as there's a space tradeoff.  It is not a strong objection, 
just a mild preference.

> I'm certainly hoping that raw-win32 is doing whatever needs to be done?
> The mentioned FSCTL_SET_SPARSE seems to be there at least.
>    

Ah, I looked at the open code and missed it.

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