One of the most important things to understand before using GRUB is how
the program refers to devices, such as system hard drives, and
partitions. This information is very important to know in order to
configure GRUB to boot multiple operating systems.
The first hard drive of a system will be called
(hd0) by GRUB. The first partition on that drive
is called (hd0,0), and the fifth
partition on the second hard drive is called
(hd1,4). In general, the naming convention for
filesystems when using GRUB breaks down in this way:
(<type-of-device><bios-device-number>,<partition-number>) |
The parentheses and comma are very important to the device naming
conventions. The
<type-of-device>
refers to whether a hard disk (hd) or floppy disk
(fd) is being specified.
The
<bios-device-number>
is the number of the device according to the system's BIOS, starting
with 0. The primary IDE hard drive is numbered
0, while the secondary IDE hard drive is numbered
1. The ordering is roughly equivalent to the way
the Linux kernel arranges the devices by letters, where the
a in hda relates to
0, the b in
hdb relates to 1, and so on.
 | Note |
|---|
| | Remember that GRUB's numbering system for devices starts at
0, and not 1. This is one
of the most common mistakes made by new GRUB users.
|
The
<partition-number>
relates to the number of a specific partition on a disk device. Like
the
<bios-device-number>,
the partition numbering starts at 0. While most
partitions are specified by numbers, if a system uses BSD partitions,
they are signified by letters, such as a or
c.
GRUB uses the following rules when naming devices and partitions:
It does not matter if system hard drives are IDE or SCSI. All
hard drives start with hd. Floppy disks start
with fd.
To specify an entire device without respect to its partitions,
simply leave off the comma and the partition number. This is
important when telling GRUB to configure the MBR for a particular
disk. For example, (hd0) specifies the first
device and (hd3) specifies the fourth device.
If a system has multiple drive devices, it is very important
to know the drive order according to the BIOS. This is rather
simple to do if a system has only IDE or SCSI drives, but if there
is a mix of devices, it can become confusing.
When typing commands to GRUB involving a file, such as a menu list to
use when allowing the booting of multiple operating systems, it is
necessary to include the file immediately after specifying the device
and partition. A sample file specification to an absolute filename is
organized as follows:
(<type-of-device><bios-device-number>,<partition-number>)/path/to/file |
Most of the time, a user will specify files by the directory path on
that partition plus the filename. This method is straightforward.
It is also possible to specify files to GRUB that do not actually
appear in the filesystem, such as a chain loader that appears in the
first few blocks of a partition. To specify these files, it is
required to provide a blocklist, which tells
GRUB, block by block, where the file is located in the partition. As a
file can be comprised of several different sets of blocks, there is a
specific way to write blocklists. Each file's section location is
described by an offset number of blocks and then a number of blocks
from that offset point, and the sections are put together in a
comma-delimited order.
Consider the following blocklist:
This blocklist tells GRUB to use a file that starts at the first block
on the partition and uses blocks 0 through 49, 99 through 124, and
199.
Knowing how to write blocklists is useful when using GRUB to load
operating systems that use chain loading, such as Microsoft
Windows. It is possible to leave off the offset number of blocks if
starting at block 0. As an example, the chain loading file in the
first partition of the first hard drive would have the following name:
The following shows the chainloader command with a
similar blocklist designation at the GRUB command line after setting
the correct device and partition as root:
Some users are confused by the use of the term "root filesystem" with
GRUB. It is important to remember that GRUB's root filesystem has
nothing to do with the Linux root filesystem.
The GRUB root filesystem is the root partition for a particular
device. GRUB uses this information to mount the device and load files
from it, among other things.
With Red Hat Linux, once GRUB has loaded its root partition that contains the
Linux kernel, the kernel command can be executed
with the location of the kernel file as an option. Once the Linux
kernel boots, it sets its own root filesystem, and that is the one
most people associate with Linux. The original GRUB root filesystem
and its mounts are forgotten. They only existed to boot the kernel
file.
Refer to the root and kernel
commands in the Section called Commands for more information.
