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Date: Thu, 09 Sep 2010 12:02:26 -0500
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: [Qemu-devel] Re: [RFC] qed: Add QEMU Enhanced Disk format
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, qemu-devel@nongnu.org, Avi Kivity <avi@redhat.com>, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

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.

Where we should do copy-on-read is a different topic.  Really, I should 
have waited to share that feature to avoid confusing the current discussion.

Regards,

Anthony Liguori

>
> Paolo