Purpose of the EFI disk?

[Agent92]

After I made the mistake of detaching the EFI disk on one of my VM:s I noticed that it still works as usual. I can boot it and it still uses EUFI. That made me wonder what the purpose of the EFI disk is? I keep seeing in documentation that you need it in order to run OVMF but seems to work fine without it.

I also managed to setup a new CentOS machine using OVMF without an EFI disk.

 

[proxfire44]

What exactly do you mean by EFI disk?

Are you referring to the roughly ~500mb partition that mounts as /boot/efi

 

[Agent92]

In the Proxmox GUI you can add a new device under: Add -> EFI disk.

And you can read the following here: https://pve.proxmox.com/wiki/Qemu/KVM_Virtual_Machines

"In order to save things like the boot order, there needs to be an EFI Disk"

All though I don't seem to have a problem with it not saving the boot order after removing the EFI disk.

 

[dcsapak]

the efi disk contains the EFIVARS which also contains the boot order
if no efidisk is specified, there is a temporary given to the vm on each start of the vm

 

[Agent92]

So there isn't really a big problem to remove a EFI disk and create a new one? I will have to recreate the boot order and perhaps some other things?

 

[podgaysky]

I don’t understand why I need a separate drive for efi vars. Oracle virtual box and KVM+qemu do not use it, everything works and so...

 

[proxwolfe]

so, is it safe to remove it?

If I remove and find that I do need it after all, will I be able to re-attach it?

 

[dcsapak]

so, is it safe to remove it?

this depends on how your guest installs its boot loader... afaik windows puts the efi bootloader into $ESP/EFI/BOOTX64.efi (which is the default fallback path which should always work as a last resort)
but not all guest os' do this
e.g. debian puts it in $ESP/EFI/debian/..., and creates a efi boot entry; which is saved in the efivars

 

[proxwolfe]

Thanks! I am glad I waited with removing it.

Do you happen to know what macOS does?

Bonus question: is the EFI disk necessary (irrespective of where the guest OS stores its EFIVARS, if the VM resides on its own dedicated passthrough raw disk? (Or would the EFIVARS then be stored on the raw disk?

 

[AngryAdm]

What would one do if the EFI disk for a debian system was accidently deleted?

 

[dcsapak]

What would one do if the EFI disk for a debian system was accidently deleted?

create a new one and recreate your boot entries with e.g. efibootmgr