[Qemu-devel] Re: [RFC] qed: Add QEMU Enhanced Disk format

[Avi Kivity]

 On 09/09/2010 08:02 PM, Anthony Liguori wrote:
> On 09/09/2010 11:48 AM, Paolo Bonzini wrote:
> > On 09/09/2010 02:49 PM, Anthony Liguori wrote:
> > > We should optimize for the future. That means a btrfs file system 
> > > and/or
> > > enterprise storage.
> >
> > So we should just implement a copy-on-read wrapper that generates a 
> > sparse raw image and uses FIEMAP (or whatever it is called these 
> > days) to test for the presence of extents.  Then you let btrfs handle 
> > everything else...
>
> My position is that we'll need a sparse image format well into the 
> future because while btrfs may be ubiquitous as a file system, IRL, 
> people transfer images around all of the time through dumb transports 
> like HTTP and fat-formatted USB keys.  A 100GB image with 1GB 
> allocated cannot explode to 100GB just because HTTP is a dump transport.

An 'Export' and 'Upload' buttons would do the job.  For command line 
users, compressing the image will remove the unallocated extents, as 
will 'qemu-img convert -O qcow2'.

It's not as nice as having a sparse format, but on the other hand, 
performance and data integrity will be better, as well as the excellent 
snapshot support.

--
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