Qcow2 images growing far beyond what they should

kvm-virtualizationqcow2qemuubuntu-12.04

I am using ubuntu 12.04 and have seen this: https://bugs.launchpad.net/ubuntu/+source/qemu-kvm/+bug/1025244

Our qcow2 images are growing far beyond what they should… with some of them growing to as much as 600gb when they should have only 20-30gb

Others are growing more slowly but still growing… normally cow images that should have a maximum of 10Gb are now showing 20gb+ this is insane! What i mean by growing is that their virtual size should be 10gb max yet they grow far beyond that on the qcow2 file

We are thin-provisioning the disks with qemu-img

I know i can temporarily solve this by reclaiming unused space with virt-sparsify.

We will soon have a big problem when the images are too big that i can't virt-sparsify them anymore to get the space back (since double the size is needed for the temporary copy)

We do have snapshots on the machines…

My questions are:

  • Does this ONLY happen because they are thin provisioned?
  • If I preallocate them will this go away?
  • How can I avoid this? changing to raw? preallocating?
  • Any way I can reclaim that space without needing double the space?

Best Answer

Several things:

  1. When you use sparse images, they will grow when the VM writes data into the disks.
  2. A snapshot is a new sparse image, able to grow to the same size as the original. So if you take a 10Gb disk, fill it up to 10Gb, and then snapshot it, the snapshot will also be able to grow to 20Gb. In the long run, this is always a problem, which is one of the resons snapshots in production are never a good idea.
  3. If you create an image with qemu-img you give the max size in the command. The image should not be able to grow beyond that size, if it does - you're seeing a bug.
  4. To avoid image sprawl, avoid snapshots, especially long running snapshots, and whenever possible stick to raw, preallocated images.
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