Resizing qemu images with backing file

Question

We have a base image and based thereon multiple copy-on-write images (qcow). There are many pages (e.g., this SO post) that explain how to resize an existing image. However, I couldn't find an explanation on how this will work in case that I'm using base images. Will resizing (making it larger) the base image break the COW images?

Let's assume I have a base image B1 and multiple COW images C1,...,CN. If I resize the base image and then allocate the new space on B1's main drive (sda), will C1,...,CN automatically be updated too, does this break these COW images, or which steps do I need to perform there to make this new space available?

Answer 1

I finally could solve it as follow:

1. I resized the base image using qemu-img resize vmdisk.qcow 30G.

2. I resized the image that uses the base image as backing file using the same command as in 1.

3. I repartitioned the disk in the VM. As my partition layout included a swap partition, I couldn't use the growpart command to resize it. Instead, I had to remove all partitions and recreate them with changed sizes. In this way I could preserve the data in the COW images.