/boot on a separate disk partition

[andron]

The Gentoo Handbook states that when using the MBR/BIOS system, the /boot directory must be on a separate partition, but when using the UEFI system, the /boot directory is located on the root partition instead of a separate partition.

Why can't the /boot directory also be located on the root partition with the MBR/BIOS system? Why is the /boot directory placed on a separate partition?

[pietinger]

When using UEFI (and GPT) you can boot everything located inside the ESP (efi system parition) ... a bootmanager/-loader (like grub) or directly a Linux kernel (or Windows ).

You have to know that grub consists of two parts. The first part is located in the ESP and is able to access the root partition and loads its second part from /boot (and then the kernel from /boot).

When using old BIOS boot (MBR) you cannot boot a kernel directly. You must use a bootmanager/-loader. And we have also two parts of grub (simplified). The first one is located in the MBR (very small) and needs a special boot partition (*) to load its second part. So, grub is the reason for this extra partition.

*) Instead this boot partition you can use also free space of your harddisk for this 2nd part of grub ... I dont recommend ...

-> https://wiki.gentoo.org/wiki/GRUB#BIOS_with_MBR

[Goverp]

IIRC my old BIOS/MBR setup had no separate boot partition. Yes, you need definitely need some space for GRUB modules between the MBR and the first partition, and this is provided by modern Linux partitioning tools that make the first partition start at +2 MB (the GRUB manual says that's currently more than enough). Those modules are placed outside the normal partitions so that they won't get moved by file system tools, and the GRUB code that gets put into the MBR relies on their absolute disk addresses to locate them.

On BIOS/GPT setups, apparently you can use partitioning to reserve the appropriate space for GRUB, but that's a different setup.

The relevant AMD64 Handbook section contains a valid partition setup, but the important point is the fact that /dev/sda1 starts at +2 MB, not that it's a /boot partition. I'm pretty sure it still works OK without a /boot partition.

Of course, one reason for using a /boot partition is that you can use a different filesystem to your Linux root partition - in my case, the latter is F2FS with extended attributes (not supported by GRUB), so I have an ext4 /boot partition. Another reason is to allow you to move or change filesystems on your Linux root; if you have a separate /boot, that will save you having to tinker with GRUB settings to handle the new partition.

[andron]

It seems to me that all of the above reasons are irrelevant to reality. It seems to me that the main reason for placing /boot in a separate partition is that old BIOS/MBR systems may not be able to read large disks, so /boot is placed in a small separate partition at the beginning of the disk.

[pingtoo]

It seems to me that all of the above reasons are irrelevant to reality. It seems to me that the main reason for placing /boot in a separate partition is that old BIOS/MBR systems may not be able to read large disks, so /boot is placed in a small separate partition at the beginning of the disk.

You are correct

But how do you arrive this conclusion? It seems that at your very first post you are not aware of this. The reason I ask this is because it seems to me you may have mixed understanding on what is "boot partition" vs "boot directory".

[Chiitoo]

The Gentoo Handbook states that when using the MBR/BIOS system, the /boot directory must be on a separate partition, but when using the UEFI system, the /boot directory is located on the root partition instead of a separate partition.

Could you share which page exactly you found this information on, please?

It certainly is not a a "must", as far as I know, though I suppose GRUB could have some specific requirements I'm unaware of.

[andron]

Could you share which page exactly you found this information on, please?

https://wiki.gentoo.org/wiki/Handbook:A ... ing_scheme

[andron]

You are correct
But how do you arrive this conclusion? It seems that at your very first post you are not aware of this.

I have read this topic:
https://forums.gentoo.org/viewtopic-p-6681609.html

[pingtoo]

You are correct
But how do you arrive this conclusion? It seems that at your very first post you are not aware of this.

I have read this topic:
https://forums.gentoo.org/viewtopic-p-6681609.html

Thank you for confirmation.

So do you need more information on why "boot partition" for MBR/BIOS need exist?

[andron]

So do you need more information on why "boot partition" for MBR/BIOS need exist?

If you know anything else, please tell me.

[pingtoo]

So do you need more information on why "boot partition" for MBR/BIOS need exist?

If you know anything else, please tell me.

I think Goverp and pietinger explained pretty good. I am just not sure if you need additional information for example does the partition need to be in special location/format/file system type etc..

[NeddySeagoon]

andron,

The history you need to understand is on the wiki.

[pietinger]

[...] The history you need to understand is on the wiki.

When you read about grub2 somewhere, you need to know that there used to be the classic grub. Then a new version was developed that was not compatible, grub2. Once the classic grub had been buried, grub2 was renamed grub. So grub2 is what we know as grub today.

[Chiitoo]

Could you share which page exactly you found this information on, please?

https://wiki.gentoo.org/wiki/Handbook:A ... ing_scheme

Thanks!

Most of that seems like an example to me, not something that is saying that a boot partition is required.

Note also, that under 'What is the BIOS boot partition?' it says:

"It is not required when booting in EFI/UEFI mode, and also not required when using a MBR/Legacy DOS disklabel."

I started with a separate partition back in 2010 myself, as it was in the example then, too, but it didn't say it was required then either as far as I can see:

https://web.archive.org/web/20101117162 ... t=1&chap=4

(The wiki as we know it now came to be back in 2014, hence the archive-seeing.)

I still use it on the main machine, which basically uses the same install from 2010, just moved from machine to another, but I also have a bunch of installs without a separate 'boot'. I feel like I have some using a GPT disklabel even, but not entirely sure at this time.

But yeah, there might be some hardware reasons for it, that it just doesn't work, but I haven't bumped into those, and other than that, it's up to the user as far as I can tell.

I do like being able to keep it unmounted aside from when updating the kernel (I use LILO by the way; never got comfortable with GRUB2 still).

[NeddySeagoon]

andron,

The original reason for a small separate /boot at the start of the drive, it had to be at the start, was the early BIOS could only read the first 520MB of the drive and BIOS calls were used to load the boot loader, kernel and initrd.
That was the first time that HDD growth outstripped the BIOS ability to read them.

The small separate /boot at the start of the drive keeps all the files needed to boot in an area where the BIOS can read them.

When the BIOS can read the entire drive, a separate /boot partition is not required.