Re: How to boot a memory stick image of a hard drive?

[Pascal Hambourg]

[I am writing only about the current GRUB version aka GRUB 2, not GRUB 
legacy]

Le 15/07/2017 à 18:01, address@hidden a écrit :
>      I notice that when I partition using fdisk is usually leaves some 
> blocks at the beginning. I guess this is for grub?

No, it is primarily for alignment with physical blocks.
But GRUB can use that space on MBR/DOS disks (not GPT).

>      In any case I 'should' be able to prepare the disk using fdisk and 
> then load my /dev/sda2 image where is should be. Make the swap space and 
> run grub-install.
>
>      I now have a running disk. So I will try moving the beginning of 
> partition 1 and reformating it as swap and see if grub 2.02 will just work.
>
>      Does grub document how much space it needs?

If it does, I am not aware of it. GRUB's core image was designed to fit 
in the 31 KiB space left by traditional CHS alignment. This space is 
generally enough when /boot/grub is in an ext filesystem on a regular 
partition. For example on my system the core image size is 25 KiB. But 
the core image can grow bigger if it needs to embed extra modules such 
as btrfs, LVM, RAID or crypto in order to read /boot/grub. But that is 
not a problem with modern 1 MiB alignment.

You can see the core image size with the size of the file 
/boot/grub/i386-pc/core.img.

> I guess there really ought to be a special grub partition.

There is a special partition type "BIOS boot" in GPT that GRUB can use 
to store the core image, but not in MBR/DOS. Also recent GRUB versions 
can install the core image in a reserved area of a btrfs partition.

Otherwise, the core image can be written as a standard file (core.img) 
but it is only possible at certain conditions and is not considered as 
reliable.

>      I made two memory stick versions. One with just /dev/sda2 and one 
> with the whole hard drive image. I think I can now get the /dev/sda2 
> image running and also update grub at the same time.