Ensure only RAW images in cache(imgconv/raw) while keeping the original image

[unsuman]

Now when a fresh image is downloaded and if it's a non-ISO image, and the image type not supported by the driver it is then converted to RAW in the cache(imgconv/raw) and a raw.digest file is created. It also works for existing images in the cache that are not supported by the driver

Fixes #4056

[unsuman]

Are RAW images not supported on Windows?

[AkihiroSuda]

Are RAW images not supported on Windows?

https://learn.microsoft.com/en-us/windows/win32/fileio/sparse-file-operations says:

It is up to the application to maintain sparseness by writing zeros with FSCTL_SET_ZERO_DATA.

Maybe it is not implemented in qemu-img.exe ?

[unsuman]

Maybe it is not implemented in qemu-img.exe ?

Yes seems so, as you pointed out

Is a .img file qcow2 or raw? I checked it on MacOS it shows raw, does it behave differently on Windows?

[AkihiroSuda]

Is a .img file qcow2 or raw? I checked it on MacOS it shows raw, does it behave differently on Windows?

Can't predict the format from the extension

[AkihiroSuda]

https://github.com/lima-vm/lima/actions/runs/22440179260/job/64980256758?pr=4637

qemu-img: SetEndOfFile error: There is not enough space on the disk.

Does --preallocation flag work for this on Windows?
https://qemu-project.gitlab.io/qemu/tools/qemu-img.html

[unsuman]

https://github.com/lima-vm/lima/actions/runs/22440179260/job/64980256758?pr=4637

qemu-img: SetEndOfFile error: There is not enough space on the disk.

Does --preallocation flag work for this on Windows? https://qemu-project.gitlab.io/qemu/tools/qemu-img.html

Gemini tells me:

The --preallocation=off flag is actually the default behavior when extending a file's size. On Linux and macOS, this works perfectly because the underlying system call (ftruncate) naturally creates a "hole" (a sparse file) without consuming physical blocks.

However, on Windows, QEMU translates this resize request into the Win32 API call SetEndOfFile.
Windows NTFS is extremely strict. If a file has not been explicitly branded with the FSCTL_SET_SPARSE attribute before SetEndOfFile is called, Windows assumes you want real, contiguous physical space.

[unsuman]

Current flow:

- Initial state. No cache here.
- vz driver requires https://example.com/foo.qcow2
- Lima downloads the qcow2 image
- Lima converts the qcow2 to the raw cache(imgconv/raw)
- Lima returns the cached raw image
- qemu driver requires the same URL
- +++ Lima returns the qcow2 image because the driver supports it

- User prunes the cache
- qemu driver requires the same URL again
- Lima downloads the qcow2 image
- Lima returns the cached qcow2 image, without any conversion
- vz driver requires the same url
- Lima converts the .qcow2 image to raw and caches it for future use

Only left with adding tests to pkg/downloader/downloader_test.go, I'll complete it before our meeting #4678

Also need to update internals.md

[jandubois]

I improved my review agent to let subagents run more multiple passes as long as they keep finding significant additional issues.

More comprehensive report: Deep Review: 20260327-223819-pr-4637