grub2: BIOS installation
4.4 BIOS installation
=====================
MBR
===
The partition table format traditionally used on PC BIOS platforms is
called the Master Boot Record (MBR) format; this is the format that
allows up to four primary partitions and additional logical partitions.
With this partition table format, there are two ways to install GRUB: it
can be embedded in the area between the MBR and the first partition
(called by various names, such as the "boot track", "MBR gap", or
"embedding area", and which is usually at least 31 KiB), or the core
image can be installed in a file system and a list of the blocks that
make it up can be stored in the first sector of that partition.
Each of these has different problems. There is no way to reserve
space in the embedding area with complete safety, and some proprietary
software is known to use it to make it difficult for users to work
around licensing restrictions; and systems are sometimes partitioned
without leaving enough space before the first partition. On the other
hand, installing to a filesystem means that GRUB is vulnerable to its
blocks being moved around by filesystem features such as tail packing,
or even by aggressive fsck implementations, so this approach is quite
fragile; and this approach can only be used if the '/boot' filesystem is
on the same disk that the BIOS boots from, so that GRUB does not have to
rely on guessing BIOS drive numbers.
The GRUB development team generally recommends embedding GRUB before
the first partition, unless you have special requirements. You must
ensure that the first partition starts at least 31 KiB (63 sectors) from
the start of the disk; on modern disks, it is often a performance
advantage to align partitions on larger boundaries anyway, so the first
partition might start 1 MiB from the start of the disk.
GPT
===
Some newer systems use the GUID Partition Table (GPT) format. This was
specified as part of the Extensible Firmware Interface (EFI), but it can
also be used on BIOS platforms if system software supports it; for
example, GRUB and GNU/Linux can be used in this configuration. With
this format, it is possible to reserve a whole partition for GRUB,
called the BIOS Boot Partition. GRUB can then be embedded into that
partition without the risk of being overwritten by other software and
without being contained in a filesystem which might move its blocks
around.
When creating a BIOS Boot Partition on a GPT system, you should make
sure that it is at least 31 KiB in size. (GPT-formatted disks are not
usually particularly small, so we recommend that you make it larger than
the bare minimum, such as 1 MiB, to allow plenty of room for growth.)
You must also make sure that it has the proper partition type. Using
GNU Parted, you can set this using a command such as the following:
# parted /dev/DISK set PARTITION-NUMBER bios_grub on
If you are using gdisk, set the partition type to '0xEF02'. With
partitioning programs that require setting the GUID directly, it should
be '21686148-6449-6e6f-744e656564454649'.
*Caution:* Be very careful which partition you select! When GRUB
finds a BIOS Boot Partition during installation, it will automatically
overwrite part of it. Make sure that the partition does not contain any
other data.
Info Catalog
grub2: Device map
grub2: Installation