Out of Space

[Oliver Grawert]

hi,
On Mi, 2016-08-10 at 13:05 -0400, Gene Heskett wrote:
> On Wednesday 10 August 2016 07:02:49 Oliver Grawert wrote:
> 
> > it definitely has not ... thats a bootloader thing ...
> > 
> > ciao
> > 	oli
> 
> Oli, I think that needs an explanation.
> ...
> 
> So please shine a light in that crevice holding the secret sauce
> that 
> makes all this work.
> 
> I am sure I am not the only one with a visible curiosity bump, :)
>  
well, the point here is that the BIOS never actually accesses your
partition table (no matter what kind of partition table that is) 

let us take a look at the MBR and the boot process a BIOS does:

wether you use MSDOS or GPT, the following is identical for both:

a disk has an MBR that is 512 bytes big and can contain the first stage
bootloader starting at byte 0 of your disk (usually with grub that is
the "stage1" binary blob) and ending at byte 440 ... your BIOS will
look if it finds a bootloader signature in the first bytes of the HDD
... and fire it up in case it finds the desired byte signature ... 

so now you are in your bootloader, past the BIOS...

lets take a look at the rest of these 512 bytes of the MBR:

in case of an MSDOS partition table you have 4 entries each 16 bytes
big starting at byte 446 ... each of these 16byte blocks is an entry
for a possible primary (or extended) partition ...

in case of a GPT partition table your partiton table actually starts in
the first logical block *after* the 512 bytes the MBR occupies, the
byte 446 to 510 block that normally holds the MSDOS table is actually
just marking the disk as "occupied" so that the disk cant be trashed if
you use a non GPT partitioning tool on it (you can easily try it by
running something like fdisk on a system with GPT). along with that
there is always a safety copy of your GPT at the very end of the disk.

in either case, the BIOS only looks at the MBR and is not dealing with
the partition table at all, your bootloader (and later in the boot the
kernel) does that.

that said ... if you use a dual boot with windows it will only boot
from a GPT disk when there is UEFI ... the same thing is true if you
use secure boot, there GPT and UEFI are tied together. so in both of
these cases the claim that you need UEFI for GPT is true.

but if you have a plain linux only machine it does not matter if you
use UEFI or BIOS, you can defintely use a GPT instead of an MSDOS
partiton table even with a BIOS based machine (at least when using a
bootloader that can understand GPT like grub2).

ciao
	oli

 
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