Re: [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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https://bugs.launchpad.net/bugs/1025244

Title:
  qcow2 image increasing disk size above the virtual limit

Status in QEMU:
  New
Status in “qemu-kvm” package in Ubuntu:
  Triaged

Bug description:
  Using qemu/kvm, qcow2 images, ext4 file systems on both guest and host
   Host and Guest: Ubuntu server 12.04 64bit
  To create an image I did this:

  qemu-img create -f qcow2 -o preallocation=metadata ubuntu-pdc-vda.img 
10737418240 (not sure about the exact bytes, but around this)
  ls -l ubuntu-pdc-vda.img
  fallocate -l theSizeInBytesFromAbove ubuntu-pdc-vda.img

  The problem is that the image is growing progressively and has
  obviously no limit, although I gave it one. The root filesystem's
  image is the same case:

  qemu-img info ubuntu-pdc-vda.img
   image: ubuntu-pdc-vda.img
   file format: qcow2
   virtual size: 10G (10737418240 bytes)
   disk size: 14G
   cluster_size: 65536

  and for confirmation:
   du -sh ubuntu-pdc-vda.img
   15G ubuntu-pdc-vda.img

  I made a test and saw that when I delete something from the guest, the real 
size of the image is not decreasing (I read it is normal). OK, but when I write 
something again, it doesn't use the freed space, but instead grows the image. 
So for example:
   1. The initial physical size of the image is 1GB.
   2. I copy 1GB of data in the guest. It's physical size becomes 2GB.
   3. I delete this data (1GB). The physical size of the image remains 2GB.
   4. I copy another 1GB of data to the guest.
   5. The physical size of the image becomes 3GB.
   6. And so on with no limit. It doesn't care if the virtual size is less.

  Is this normal - the real/physical size of the image to be larger than
  the virtual limit???

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