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

[Daniel P. Berrange]

On Wed, Aug 25, 2010 at 09:26:04AM -0500, Anthony Liguori wrote:
> 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.

QCow2 is also useful with NFS because the encryption capabilities
let the virt host administrators protect guest data from the
storage administrators.

Regards,
Daniel
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