Re: Booting an old PC to SSD that the BIOS cannot see

[Pascal Hambourg]

Le 30/12/2020 à 08:08, Glenn Washburn a écrit :
> On Tue, 29 Dec 2020 15:33:50 -0800
> Robert Furber via Support requests for the GRand Unified Bootloader
> <help-grub@gnu.org> wrote:
(...)
> I don't know a bunch about NVMe, but from what I do know, its not so
> much that NVMe needs to be initialized, and then the "system" can use
> it. Its that NVMe has its own protocol (communication language), which
> grub would need to know how to speak to get access to the data. So grub
> can not chainload to an NVMe drive that is not recognized by the
> BIOS/UEFI.

This is not different from any other kind of hardware.

> > Presumably I can put /boot on /dev/sda and '/' and '/home' on
> > /dev/nvme.. as per Chris Greene, if I understand correctly?
>
> Yes. Another option could be to use a USB flash drive as a boot device,
> if you don't want the spinning harddrive in the machine at all
> (assuming your bios is capable of USB boot).

IME, booting from USB can be painfully slow on some machines, especially 
with some USB 2.0 (high-speed) flash drives. Maybe it is better with USB 
3 (super-speed).

If you are looking for high boot speed, low noise and low consumption, 
an option is to boot from a cheap SATA SSD.

> If you really want /boot on the NVMe, you could boot to a special
> kernel+initrd on a USB/hd which is configured to find the kernel and
> initrd on the NVMe and kexec into it.

I am not sure you will save much time doing this, unless the initial 
kernel and initramfs are really stripped down and the final ones are 
bloated. Loading the average kernel and initramfs from a hard drive 
usually takes less than one second.

> > Even so, booting done by Grub & Co. is a ton of work, consuming about
> > 30s. Would there be any way to speed up booting time by doing part of
> > the initialization from /dev/sda, the HDD and the heavy lifting from
> > /dev/nvme.., sthe SSD?

The heavy job is done after the kernel and initramfs have been loaded by 
GRUB, and does not use /boot.