[Qemu-devel] [Bug 1025244] Re: qcow2 image increasing disk size above the virtual limit

[Stefan Hajnoczi]

On Tue, Dec 18, 2012 at 10:18:20AM -0000, Andy Menzel wrote:
> Any solution right now? I have a similar problem like Todor Andreev;
> Our daily backup of some virtual machines (qcow2) looks like that:
> 
> 1. shutdown the VM
> 2. create a snapshot via: "qemu-img snapshot -c nameofsnapshot..."
> 3. boot the VM
> 4. backup the snapshot to another virtual disk via: "qemu-img convert  -f qcow2 -O qcow2 -s nameofsnapshot..."
> 5. DELETE the snapshot from VM via: qemu-img snapshot -d nameofsnapshot...

It's not safe to modify the qcow2 file while the guest is running.  This
means Step 5 is not really safe and could result in an inconsistent
image.

This may also be causing the problem: the QEMU process has a variable
with the next free cluster index.  Since Step 5 runs as a separate
process it does not update the QEMU process' next free cluster index
variable.  QEMU doesn't know that there are now free clusters within the
image file because you updated the file behind QEMU's back - the result
is that it grows the file.

Please try deleting the last backup snapshot between Step 1 and Step 2.
This way you'll free the space while QEMU isn't accessing the image
file.  When you boot up the image file again QEMU should reuse the freed
clusters.

Stefan

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Title:
  qcow2 image increasing disk size above the virtual limit

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