feat: add option to use zstd compression for disk.img file

[dustdfg]

I confirm this feature has not been previously requested

• I have searched the issues and this feature has not previously been requested

Is your feature request related to a problem? Please describe.
File size is smaller and probably it will be faster

Describe the solution you'd like
I'd like too see an option in the config file or cli argument for quickemu which will allow to create disk.img file which uses zstd compression. Maybe even use zstd by default

Describe alternatives you've considered
You can use ./qemu-img convert -f qcow2 -O qcow2 -c -o compression_type=zstd input_file output_file after file was created but it is manual work...

[flexiondotorg]

Thanks for raising this. While researching compression options, we discovered something important about how qcow2 compression actually works that's worth sharing.

Compression in qcow2 is read-only. When you compress an image with qemu-img convert -c, that compression only persists until the guest writes to those areas. Any cluster that gets modified is rewritten uncompressed, and QEMU doesn't re-compress it. Over time, a compressed image will grow back towards its uncompressed size as the guest uses it.

From the QEMU qemu-img documentation:

"The compression is read-only. It means that if a compressed sector is rewritten, then it is rewritten as uncompressed data."

This is also described in the qcow2 specification - compressed clusters require copy-on-write and cannot be modified in place.

For ongoing disk space efficiency, #1825 (now merged) takes a different approach: enabling discard=unmap and detect-zeroes=unmap on all system drives. This allows the guest's TRIM requests to flow through to the qcow2 layer, reclaiming space continuously as files are deleted - rather than relying on one-time compression that degrades with use.

That said, zstd is genuinely faster than zlib for both compression and decompression, so if you need to shrink an image for archival or distribution, you can use:

qemu-img convert -c -O qcow2 -o compression_type=zstd source.qcow2 dest.qcow2

Closing this as the discard optimisations in #1825 address the underlying goal of keeping disk images trim.