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[SOLVED] Quick question regarding the partition scheme
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Rozza
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PostPosted: Mon Jun 18, 2018 3:02 am    Post subject: [SOLVED] Quick question regarding the partition scheme Reply with quote

Hey everyone,

So after my previous post I decided to dive into gentoo. I only have one quick question regarding the default partition scheme. I am going to be booting in UEFI so I know I need a EFI/boot partition, however, the handbook says there should also be a BIOS boot partition (only about 1-2MB). Is this really necessary? On my previous arch install I had the drive partitioned into EFI, swap, root, and home. Will this format be appropriate, or is it recommended to have the bios partition added as well?


Last edited by Rozza on Mon Jun 18, 2018 10:48 am; edited 1 time in total
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The Doctor
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PostPosted: Mon Jun 18, 2018 3:04 am    Post subject: Reply with quote

Well, there is no such thing as a default install.

The BIOS partition is intended for grub2 to live. If you wish to use a different boot up scheme then modify accordingly.

You can really do as you see fit. I like using the built in firmware so no need for anything special, just a FAT boot partition.
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Rozza
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PostPosted: Mon Jun 18, 2018 3:10 am    Post subject: Reply with quote

The Doctor wrote:
Well, there is no such thing as a default install.

The BIOS partition is intended for grub2 to live. If you wish to use a different boot up scheme then modify accordingly.

You can really do as you see fit. I like using the built in firmware so no need for anything special, just a FAT boot partition.


Thanks for the reply! I was under the impression that grub2 can live inside the efi boot partition? I am most likely wrong though
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The Doctor
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PostPosted: Mon Jun 18, 2018 3:15 am    Post subject: Reply with quote

I don't use grub2 so my understanding could very well be off here, but what I think happens is that small partition is the FAT partition and the /boot is free to be an ext* or whatever. Personally I like putting the kernel on the FAT partition and calling that /boot with the system firmware but again this is all about choice. How you approach this problem is completely up to you.

I would read through the handbook and see what you want to do and what options you want. That may illuminate your way. Everything here is a choice and there is no right or wrong one. Only forks in the road.
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Marcih
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PostPosted: Mon Jun 18, 2018 11:40 am    Post subject: Reply with quote

Your understanding is off, as you said, The Doctor. :) The BIOS boot partion is exactly that, a boot partition for BIOS. It is indeed where GRUB lives, but it is only a requirement for "legacy" BIOS systems when combined with GPT-formatted drives.

Before the wonders of GPT, UEFI, Secure Boot and other such inventions, the standard was pretty much what IMB had in their PC and everyone just went with that ("IBM PC-compatible" ring a bell?). This included how the data on the disk should be layed out and how the BIOS should behave, amongst other things of course. The disk was to have a boot sector, a very small space (a single sector on the disk?) between the start of the disk and the first partition from which the BIOS would read. This was called the Master Boot Record (there were other things in the MBR like information on how partitions are organised or the filesystems of those partitions, so the MBR wasn't just the boot sector) hence the name of the partition table. The bootloader (or code that would load it if the bootloader itself were too big) would therefore reside in the MBR because that's what the BIOS expected and that was the only way to boot. GRUB of course had to do the same, i.e. copy the bootloader to the MBR.
Jumping back to present day and the Unified Extensible FIrmware Interface (UEFI) exist, meant to replace the old IMB standards. The GUID Partitioning Table (GPT) is part of the new standard and meant to replace MBR. Since UEFI doesn't use a boot sector, there is no real eqivalent in GPT (there seems to be space reserved at the beginning where the MBR would've been, can anyone knowledgeable confirm what this is used for? Is that where the "bios_grub" partition is moved?). Since the BIOS is looking for a code in a place where there is none, a work-around is needed. GRUB (and other bootloaders) can reserve an unformatted partition, called the GRUB Boot Partition or "bios_grub" in GNU Parted, where it stores the code that would otherwise be written to the MBR. This allows the BIOS to read and execute the code it's looking for in order to boot, even when booting from GPT disks.

Moral of the story: The unformatted ~1MB "bios_grub" partition is only needed for users going the BIOS+GPT route and is there due to historical reasons.

Can anyone update the handbook on this (and/or move my post to the tips and tricks forum :D)? It's a shame that it doesn't clear this up as I was confused about the point of the bios_grub partition at first as well.

EDIT: Here is the part in GRUB's manual where they briefly mention it.
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Fitzcarraldo
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PostPosted: Mon Jun 18, 2018 6:01 pm    Post subject: Reply with quote

Nicely explained, Marcih. To be pedantic, GRUB partly lives on the BIOS Boot Partition.

Rozza, in case it's of any help, I wrote the following after finding the Gentoo Handbook and various other sources confusing: Partitioning hard disk drives for BIOS-MBR, BIOS-GPT and UEFI-GPT in Linux.
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NeddySeagoon
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PostPosted: Mon Jun 18, 2018 7:28 pm    Post subject: Reply with quote

Marcih,

The partition table was a dirty hack introduced when HDD got bigger than 32MB (Yep, megabytes)
The extended partition was another hack to get past 128MB.

The MSDOS partition table only has four entries because that's all there was room for at the end of the boot record.

There is a tradition with MSDOS partitions that the first partition starts at Cylinder 0, Head 1, Sector 0. Over the years, that came to mean LBA 63.
Thus there was always some spare space before the first partition.
Thats been fought over by various tools over the years, including grub, which installs its stage 1.5 there.

With GPT, this free space is gone. The partition table starts at LBA 1, so grub needs a partition to install its stage 1.5 when BIOS and GPT are used together.

With GPT, the old MSDOS partition table is still there but it contains a fake single partition entry of type 0xee, which means that GPT is in use.
If you use BIOS the MBR code is there too.
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