how can I set the extent size hint to a file with command line? 
I have google for a lot, but I can't find a answer

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> On May 30, 2016, at 13:04, Dave Chinner <david@fromorbit.com> wrote:
> 
> On Mon, May 30, 2016 at 12:45:07PM +0800, baotiao wrote:
>> This machine is running for qemu,  the file is qemu qcow file type
>> 
>> [root@w-openstack20 /data/nova/instances/7898b630-c4ef-49ab-9ce9-3735e090c282]# file disk
>> disk: QEMU QCOW Image (v3), has backing file (path /data/nova/instances/_base/3f27393376152ac352b2d85703011e38517e), 429496729600 bytes
> 
> Oh, you're using delta/snapshot based qcow images. That, by it's
> very nature, generates fragmented image files as they are a delta
> over the backing file.
> 
> ....
>> actual 48421430, ideal 20716, fragmentation factor 99.96%
>> 
>> How can I solve this problem, what do you suggest me to do?
> 
> Use a extent size hint (say 1-8MB) for your qcow2 image files so that
> they don't fragment badly as they are written to. 
> 
> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@fromorbit.com