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Subject: [Qemu-devel] Re: [RFC] qed: Add QEMU Enhanced Disk format
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To: Anthony Liguori <anthony@codemonkey.ws>
Cc: Kevin Wolf <kwolf@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>, qemu-devel@nongnu.org

  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.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.