Enable trimming QEMU disks

[chrisx8]

This would enable trimming of QEMU virtual disks, by adding the discard=on argument to -drive flag for basedisk or diffdisk. As a result, running fstrim -a in the guest would reduce the disk space consumed by QEMU disks (basedisk or diffdisk), and reduce the disk usage of Lima guests.

Current behavior: there's no easy way to reduce the disk usage of diffdisk, and running fstrim -a in the guest has no effect.

Resolves #356

[jandubois]

Thank you!

I'm so happy this actually works! 😄

[jandubois]

FYI: Alpine doesn't have fstrim -a, but requires mountpoint, e.g. fstrim /tmp. It still releases all unused space from the diffdisk.

[CzBiX]

Does basedisk really need discard too?

[chrisx8]

Looks like Lima only uses either basedisk or diffdisk, so I'd say yes.

[jandubois]

Looks like Lima only uses either basedisk or diffdisk, so I'd say yes.

We never write to basedisk though: it either is a read-only ISO, or it is the base for the diffdisk; all modifications should always go to the diffdisk unless I misunderstand how this works. So I don't think it is needed for the basedisk, but does it hurt? Is there any performance/memory cost associated with turning it on?

[chrisx8]

lima/pkg/qemu/qemu.go

Lines 435 to 439 in 020d7ee

	if diskSize, _ := units.RAMInBytes(*cfg.LimaYAML.Disk); diskSize > 0 {
	args = append(args, "-drive", fmt.Sprintf("file=%s,if=virtio,discard=on", diffDisk))
	} else if !isBaseDiskCDROM {
	args = append(args, "-drive", fmt.Sprintf("file=%s,if=virtio,discard=on", baseDisk))
	}

This code snippet suggests that only one of basedisk or diffdisk is used on the QEMU VM. If QEMU is set to boot from the diffdisk, it knows about the basedisk.

There shouldn't be any performance impact during normal use. However, fstrim may take slightly more time, although I can't really tell the difference when I was testing.

[jandubois]

Ah yes, the only time we use the basedisk directly is when it is not an ISO and the diffdisk size is 0. So everything looks correct.