Re: Do I need to act on this warning from apt when installing grub?

[Pascal]

hi,
the table has been slightly modified and some explanations have been added.
please let me know if there are any errors or gaps in this summary.
regards, lacsaP.

GRUB :

             INTEL/MBR               GPT
            PARTITIONING         PARTITIONING
      +---------------------+---------------------+
      |MBR 1/3              |MBR 1/3              |
      |POST-MBR-GAP 2/3     |                     |
      |+-------------------+|+-------------------+|
      ||PART               |||PART BIOS-BOOT 2/3 ||
      ||/boot/grub/ 3/3    ||+-------------------+|
 BIOS ||...                ||+-------------------+|
 BOOT ||                   |||PART ESP (FAT)     ||
      ||                   |||/boot/grub/ 3/3    ||
      ||                   |||...                ||
      |+-------------------+|+-------------------+|
      +---------------------+---------------------+
      |                     |                     |
      |                     |                     |
      |+-------------------+|+-------------------+|
      ||PART ESP (FAT)     |||PART ESP (FAT)     ||
      ||/boot/efi/ 1/2     |||/boot/efi/ 1/2     ||
 UEFI ||/boot/grub/ 2/2    |||/boot/grub/ 2/2    ||
 BOOT ||...                |||...                ||
      ||                   |||                   ||
      ||                   |||                   ||
      |+-------------------+|+-------------------+|
      +---------------------+---------------------+

3 steps for GRUB in BIOS mode and 2 steps in UEFI mode.

in BIOS mode :
first, BIOS load MBC (Master Boot Code - step 1/3 of GRUB) from MBR to
memory and run it
- with INTEL/MBR partitioning, GRUB will be chained from MBR, then to
POST-MBR-GAP (step 2/3) and then to /boot/grub/... (step 3/3)
- with GPT partitioning, GRUB will be chained from MBR, then to a special
partition of type BIOS-BOOT (step 2/3 - can't use POST-MBR-GAP because used
by GPT) and then /boot/grub/... (step 3/3)

in UEFI mode :
first, UEFI load an run a (GRUB) loader (step 1/2 of GRUB) located on ESP
(EFI system partition formated with FAT file system)
then the loader chain to /boot/grub/... (step 2/2)

Le jeu. 15 déc. 2022 à 11:31, Pascal <patatetom@gmail.com> a écrit :

>           INTEL/MBR           GPT
>          PARTITIONING     PARTITIONING
>       +----------------+----------------+
>       |MBR             |MBR             |
>       |POST-MBR-GAP    |GPT             |
>       |+--------------+|+--------------+|
>       ||PART          |||PART BIOS-BOOT||
>       ||/boot/grub/   ||+--------------+|
>  BIOS ||...           ||+--------------+|
>  BOOT ||              |||PART ESP (FAT)||
>       ||              |||/boot/grub/   ||
>       ||              |||...           ||
>       |+--------------+|+--------------+|
>       +----------------+----------------+
>       |                |                |
>       |                |GPT             |
>       |+--------------+|+--------------+|
>       ||PART ESP (FAT)|||PART ESP (FAT)||
>       ||/boot/grub/   |||/boot/grub/   ||
>  UEFI ||...           |||...           ||
>  BOOT ||              |||              ||
>       ||              |||              ||
>       ||              |||              ||
>       |+--------------+|+--------------+|
>       +----------------+----------------+
>
> Le mer. 14 déc. 2022 à 12:36, Goh Lip <g.lip@gmx.com> a écrit :
>
>>
>>
>> On 12/14/22 17:15, Chris Green wrote:
>> > On Wed, Dec 14, 2022 at 10:31:40AM +0800, Goh Lip wrote:
>> >>
>> >>
>> >> On 13/12/2022 18:55, Chris Green wrote:
>>
>> >>
>> >> Is your partition table GPT, not msdos?
>> >>
>> > Yes, it's GPT:-
>> >
>> >      Disk /dev/sda: 931.51 GiB, 1000204886016 bytes, 1953525168 sectors
>>
>> >      Disklabel type: gpt
>>
>> >
>> >      Device       Start        End    Sectors  Size Type
>> >      /dev/sda1     2048       4095       2048    1M BIOS boot
>> >      /dev/sda2     4096    1054719    1050624  513M EFI System
>> >      /dev/sda3  1054720 1953523711 1952468992  931G Linux filesystem
>> >
>> >
>> > (There's an 8Tb USB drive as well, this is my backup system which
>> > lives in the garage, quite a way away from the house)
>> >
>>
>>
>> See
>> https://www.gnu.org/software/grub/manual/grub/grub.html#BIOS-installation
>>
>> To set up bios-legacy boot on gpt partitioning properly, you will need a
>> small partition labelled "bios_grub".
>> And yes, without it, as you found out, it will still install and boot.
>> But it will not be reliable and will, in time, likely to fail again. And
>> you can still make it work again the way you have done (and will likely
>> fail again).
>>
>> Personally, I would suggest that for gpt partitioning, boot in uefi. And
>> in msdos partitioning, boot in bios-legacy (though that too can be
>> booted in uefi boot - with unreliable performance).
>>
>> Hope this explains. Regards.
>>
>> ps: best when issuing grub-install command, use "--target=i386-pc" in
>> that command for boot-legacy boots and "--target=x86_64-efi" for uefi
>> boots, especially when installing legacy in gpt or uefi in msdos (latter
>> always not recommended, but I've done it)
>>
>>
>>
>>