Converting qcow2 images to raw is too slow

[nirs]

Converting qcow2 images to raw is too slow

[AkihiroSuda]

qemu-img is efficient, supports all images formats, well maintained, and available everywhere.

On macOS, it is hard to install qemu-img when Homebrew/MacPorts/nix is disallowed due to employers' policy

[jandubois]

Fix:

• use qemu-img convert if available

It would be better to fix the speed of the builtin conversion so it will be fast even when QEMU is not installed. Given that the default emulation in Lima 1.0 will be VZ, qemu will be an optional dependency.

[nirs]

I don't think that reinventing qemu-img good direction. The time spent on it can be spent on features that that add values to users. qemu-img is efficient, supports all images formats, well maintained, and available everywhere.

[afbjorklund]

You can default to qemu-img (where available), and then fallback to the library as a slower fallback option?

We have used this trick elsewhere, like with SFTP or with XZ. The downside is having two code paths to test...

[AkihiroSuda]

This may have a room for optimization
https://github.com/lima-vm/go-qcow2reader/blob/v0.1.2/image/qcow2/qcow2.go#L795-L800