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[Qemu-devel] [Bug 1025244] Re: qcow2 image increasing disk size above th

From: Thomas Huth
Subject: [Qemu-devel] [Bug 1025244] Re: qcow2 image increasing disk size above the virtual limit
Date: Wed, 04 Apr 2018 17:52:23 -0000

Looking through old bug tickets... is there anything left to do here? Or
should we rather close this ticket nowadays?

** Changed in: qemu
       Status: New => Incomplete

** Changed in: qemu-kvm (Ubuntu)
       Status: Triaged => Incomplete

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  qcow2 image increasing disk size above the virtual limit

Status in QEMU:
Status in qemu-kvm package in Ubuntu:

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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