we are using kvm/qemu with qcow2-images for our virtual machines.
qcow2 has this nice feature where the image file only allocates the actually needed space by the virtual-machine. but how do i shrink back the image file, if the virtual machine's allocated space gets smaller?
1.) i create a new image with qcow2 format, size 100GB
2.) i use this image to install ubuntu. installation needs about 10 gb, the image-file grows up to about 10GB. nothing unexpected so far.
3.) i fill up the image with about 40 GB of additional data. the image-file grows up to 50GB. i am ok with that :-)
4.) this is where it gets strange: i delete all of the 40GB data on the image, but the image-size still eats up 50GB.
question: how do i free up that 40GB of data and shrink the image to the only needed 10 GB?
thanks in advance, berni