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?

[nirs]

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

The default format for what? For additional disks?

Yes

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?

With the goqcow2reader we cannot write qcow2 images, since the API accepts a io.ReaderAt, and of course the module name make it clear that this is read only. We can modify the code to support writing empty qcow2 image, but why do we want to do this?

For vz there is no need for creating qcow2 disks. For qemu vms you must install qemu, so you already have qemu-img.