Hard Disk Installation

For a booting Linux on the hard disk there are basically the following choices:

• Use an OS neutral MBR boot sector program (such as the one from DOS) and let it boot a Linux boot loader in a primary partition. Some such boot sector programs let the user select one of several operating systems at boot time.
• Boot a Linux boot loader (LILO or GRUB) directly from the MBR.
• Chainload a Linux boot loader through the boot loader of another OS (the NT Boot Manager). One could even chainload Linux from another instance of a Linux boot loader. There could be an instance of GRUB that is booted from the MBR and GRUB could chainload several instances of LILO, each on its own partition.
• Boot Linux from a diskette. The diskette may contain just the boot sector (it loads the second stage of the boot loader from the hard disk), the entire boot loader or even the boot loader plus the kernel.
• Boot Linux through DOS using LOADLIN.

Which option you use, depends on several factors:

• What other operating systems run on the same machine? Chainloading Linux from the Windows NT boot loader would make no sense on a machine without NT.
• What partitions are used on the disk? Is a RAID used? As far as I know, LILO is the only Linux boot loader that boots from a RAID.
• What is the relative importance of each operating system?
• Perceived reliability of each boot loader.
• Personal preferences.

One factor that may complicate boot loader installation is that each operating system has its own naming and numbering scheme for hard disks and partitions and so does GRUB. The BIOS assigns a number to each disk. The first disk is 0x80, the second disk is 0x81, etc. When you install LILO, things go without problems most of the time, but there are situations in which it may be necessary to specify the BIOS code of a drive.

The naming convention of hard disks in GRUB can be confusing:

• The whole hard disk (if you want to install GRUB in the MBR) is called (hd0), (hd1) etc.
• Partitions are counted from zero, so partition 1 on the first disk is called (hd0,0). Extended partitions are counted from 4, like they are counted from 5 in Linux.
• Disks are also counted from zero in the order that BIOS assigns them. On most systems /dev/hda corresponds to (hd0) (except on systems with only SCSI disks), but what Linux disk will correspond to (hd1), depends on what you connect to the EIDE ports. If the primary slave is a hard disk, this disk will be /dev/hdb in Linux and (hd1) in GRUB. If the primary slave is a CD-ROM and the secondary master is a hard disk, the hard disk will be /dev/hdc in Linux and (hd1) in GRUB.

Table 2 shows the disk and partition numbers on a system with two hard disks, connected as master and slave to the primary IDE controller. The first disk contains two primary partitions (DOS and Linux) and two logical partitions (DOS and Linux swap). The second disk contains one primary DOS partition.

Table 2: Disk and partition names (first example)

Disk	Linux	BIOS code (LILO)	GRUB disk name	DOS drive
IDE Primary Master	/dev/hda	0x80	(hd0)	-
Primary DOS	/dev/hda1	-	(hd0,0)	C:
Primary Linux	/dev/hda2	-	(hd0,1)	-
Extended	/dev/hda3	-	(hd0,2)	-
Logical DOS	/dev/hda5	-	(hd0,4)	E:
Linux swap	/dev/hda6	-	(hd0,5)	-
IDE Primary Slave	/dev/hdb	0x81	(hd1)	-
Primary DOS	/dev/hdb1	-	(hd1,0)	D:

Table 3 shows the disk and partition numbers on a system with three hard disks, one on the primary IDE controller and one on the secondary IDE controller and one SCSI.

Table 3: Disk and partition names (second example)

Disk	Linux	BIOS code (LILO)	GRUB disk name	DOS drive
IDE Primary Master	/dev/hda	0x80	(hd0)	-
Primary DOS	/dev/hda1	-	(hd0,0)	C:
Primary Linux	/dev/hda2	-	(hd0,1)	-
IDE Secondary Master	/dev/hdc	0x81	(hd1)	-
Primary DOS	/dev/hdc1	-	(hd1,0)	D:
First SCSI	/dev/sda	0x82	(hd2)	-
Primary DOS	/dev/sda1	-	(hd2,0)	E:

If you use FreeBSD as well, things get even more complicated. BSD divides a normal disk partition into several sub-partitions. Linux can only access them if support for BSD disk labels is compiled into the kernel. Needless to say that Linux, BSD and GRUB all have different names for these sub-partitions.

In GRUB you have to use GRUB partition names for files accessed by GRUB, but Linux partition names for arguments on the kernel command line. The following is a very typical kernel command line in GRUB:

kernel (hd0,1)/vmlinuz root=/dev/hda2

Finally on some (mostly older) systems, disk geometry may cause problems. As long as everything uses LBA, no problems are expected. If your BIOS does not support LBA, boot loaders using the BIOS have no way to reach beyond 1024 cylinders. nuni may be a solution in such a case, but most of the time you will need to create a small partition near the start of the disk and boot the kernel from there.