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Date: Fri, 10 Sep 2010 14:15:59 +0300
From: Avi Kivity <avi@redhat.com>
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Subject: Re: [Qemu-devel] [RFC] qed: Add QEMU Enhanced Disk format
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To: Christoph Hellwig <hch@lst.de>
Cc: Kevin Wolf <kwolf@redhat.com>, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>, Stefan Hajnoczi <stefanha@gmail.com>, Alexander Graf <agraf@suse.de>, qemu-devel@nongnu.org

  On 09/10/2010 12:01 AM, Christoph Hellwig wrote:
> On Thu, Sep 09, 2010 at 09:24:26AM +0300, Avi Kivity wrote:
>> The other thing we can do is defragment the logical image, then
>> defragment the underlying file (if the filesystem supports it, issue the
>> appropriate ioctl, otherwise defragment to a new file which you write
>> linearly).
> What's what the defragmentation code does in a slightly optimized
> fashion anyway - so if you want to do it from qemu just do it that
> way.  Don't even bother calling the filesystem ioctls directly given
> that they just implementa low-level helpers and the actual logic is
> in the userspace side of the defragmentation tools.

Well, if we ask the kernel do to it, we gain any future optimizations as 
well.  For example, if parts of the file are already defragmented, the 
kernel can avoid moving that data.

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