On Mon, Feb 6, 2012 at 6:12 AM, Russell L. Harris
<rlhar...@broadcaster.org> wrote:
Some GPT labelled disks need to have the boot flag set on the
"bios_grub" partition (in violation of the EFI spec) for a box to be
bootable. And some Lenovo models are affected.
> (1) It seems that everything goes back to Intel, which introduced two
> interrelated innovations:
>
> => The Globally Unique Identifier Partition Table ("GPT" or "GUID
> Partition Table") is a replacement for the old M$DO$ partition
> table.
>
> => The Extensible Firmware Interface (originally, "EFI", now
> "UEFI" in which "U" means "unified") is a replacement for the old
> Basic Input/Output System ("BIOS"), at least with respect to drive
> hardware. UEFI appears to correspond to the Advanced Host
> Controller Interface ("AHCI").
AFAIK, GPT's part of the EFI spec.
> (2) With GTP, the scheme of primary, extended, and logical partitions
> distinctions is done away. Instead, there are only two categories of
> partition; these may be termed "normal" and "bios-grub". All normal
> partitions are equal, and the bios-grub partition (of which there can be
> only one) is optional. With GPT, there may be as many as 128 normal
> partitions.
"bios_grub"/"bios_boot" is a type/flag like "boot", "lvm", "raid". In
fdisk/gdisk, you use "t" to set them and in parted you use "set".
> (3) In GPT, the purpose of the bios-grub partition, if it is present,
> is to contain bootloader code. However, inasmuch as the bootloader
> code may be stored in a normal partition, mounted as "/boot", the
> bios-grub partition is optional.
The boot loader code's on "/boot" but the PBR'll have pointers to it.
> (4) Like the MBR of the old scheme, the bios-grub partition is
> unformatted, for the partition begins at a pre-defined physical
> location. However, it appears that the bios-grub partition -- unlike
> the old MBR -- is not constrained to a specific length.
It's the first partition so I guess that you can more or less say that
it "begins at a pre-defined physical location."
I have no idea how large it can be but it doesn't need to be very big.
Fedora's the only distribution with which I'm familiar that defaults
to a GPT label and the bios_boot partition's set to 1 MB, probably for
optimal alignment of partitions.
> (5) Machines without the new UEFI/AHCI system are unable to work with
> the new GPT partition scheme.
You can use a GPT label with a BIOS box. AFAIK, you need to do so
above a certain disk size.
> (13) GParted is able to work with GPT as well as with the old scheme,
> but GParted may default to the old scheme. Therefore, in using
> GParted to implement the new partitioning scheme, take care to specify
> GPT.
I don't use GParted but it must be like parted; you must have to
create a GPT label and create an initial partition with a bios_grub
flag manually.
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