limactl depends on qemu or qemu-img, but lima brew package does not depend on qemu

[nirs]

Description

When installing lima via brew, qemu is not installed.

limactl start works with vz and create the root disk without qemu-img, but creating a vm with additional disks will fail when creating the disk:

% limactl disk create mydisk --size 10g
INFO[0000] Creating qcow2 disk "mydisk" with size 10GiB 
FATA[0000] failed to create qcow2 disk in "/Users/nsoffer/.lima/_disks/mydisk": failed to run [qemu-img create -f qcow2 /Users/nsoffer/.lima/_disks/mydisk/datadisk 10737418240]: "": exec: "qemu-img": executable file not found in $PATH 

This failure could be avoided if we remove the unneeded dependency on qemu-img (#2853), but we still depend on qemu-img to create qcow2 disks. We also depend on qemu for the limactl snapshot command.

If we have a dependency we should depend on the package we depend on for best user experience.

If we want to introduce a soft dependency, that we can work with or without, we need to change the code so qcow2 format and qemu vmType are available only if qemu-img is available. The online help can recommend installing additional packages.

Possible fixes:

• require qemu in lima brew package
• change the code to make qemu-img optional
  • make default disk format "raw"
  • support qcow2 disks only if qemu-img is available
  • support snapshots for qemu vms only if qemu is installed
  • fail validation or warn of vmType: qemu if qemu is not installed

Related issues:

• Lima has a dependency with qemu on Mac #3168 (dup of this)
• limactl disk should not depend on qemu-img for raw disks #2853

[AkihiroSuda]

require qemu in lima brew package

I don't think qemu should be depended on, as it is required only for rarely-used additional disks when vmType is set to vz.

[nirs]

require qemu in lima brew package

I don't think qemu should be depended on, as it is required only for rarely-used additional disks when vmType is set to vz.

It makes sense, so we want a soft dependency. If qemu-image is available, we can support qcow2 format when creating or resizing images. If not, trying to create qcow2 format or resize qcow2 image can fail with helpful message like:

"qcow2" format requires the qemu package

And same if you try to resize a qcow2 disk.

To make this change we need to change the default format to be raw. This may be break some users, but using a default which is not available in all case is a mistake that we have to fix sooner or later.

But if we remove the unneeded dependency on qemu-img for raw images (#2853), this issue is mostly resolved, since with vz there is no use case for creating or resizing disk in qcow2 format.

[jandubois]

To make this change we need to change the default format to be raw.

The default format for what? For additional disks?

I don't understand: if we can read qcow2 disks (to convert to raw format), why can't we also create empty/unformatted qcow2 disks?