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Date: Tue, 16 Oct 2012 08:54:46 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] delvm does not reduce the qcow2 file size
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To: Kuniyasu Suzaki <k.suzaki@aist.go.jp>
Cc: qemu-devel@nongnu.org

Il 16/10/2012 08:23, Kuniyasu Suzaki ha scritto:
>>> "delvm" is a command to delete a snapshot image in a qcow2 file.
>>> However it does not reduce the qcow2 file size.
>>
>> Note that the file will not grow when new allocations are performed in
>> the future.
> 
> You mean that the qcow2 file size will not increase when "savevm"
> commnad is issued after "delvm".
> Namely, the qcow2 file size is almost same when I take a snapshot
> after delete previous snapshot image.

Exactly.  The first writes to the qcow2 file will also reuse the space
left free by "delvm", if they have to allocate new clusters.

>> QEMU should also convert the unused clusters to "holes" in the file
>> system, but it doesn't do that yet.
> 
> I hope a qcow2 file is reduced automatically when a snapshot is deleted.

Yes, that's part of the plan.  The file would not reduce its size, but
the space would still be given back to the filesystem.

Paolo