[Qemu-devel] Re: [RFC][STABLE 0.13] Revert "qcow2: Use bdrv_(p)write_sync for metadata writes"

[Anthony Liguori]

On 08/25/2010 09:18 AM, Christoph Hellwig wrote:
> On Wed, Aug 25, 2010 at 04:23:38PM +0300, Avi Kivity wrote:
>    
> >   On 08/25/2010 03:46 PM, Anthony Liguori wrote:
> >      
> > > If we had another disk format that only supported growth and metadata
> > > for a backing file, can you think of another failure scenario?
> > >        
> > btw, only supporting growth is a step backwards.  Currently file-backed
> > disks keep growing even the guest-used storage doesn't grow, since once
> > we allocate something we never release it.  But eventually guests will
> > start using TRIM or DISCARD or however it's called, and then we can
> > expose it and reclaim unused blocks.
> >      
> Together with file level snapshots Thin Provisioning support basically
> makes qcow2 obsolete.
>    

I think we'll always need a sparse image format.  As long as people copy 
images to USB thumb drives containing vfat file systems or over dumb 
transports like HTTP, it will be necessary.

However, I agree that really advanced things like snapshotting is best 
done with a smart file system.

Regards,

Anthony Liguori

> I already have a prototype implementation of scsi discard for qemu,
> which together with the XFS extent size hints allows us to very
> efficiently using growing and shrinking images on the bare filesystem.
> XFS at this point doesn't have file-level snapshots, but I'm planning
> to port the hole punhing and extent size hints to btrfs and maybe
> ocfs2 which will give you all that.
>
>