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Chapter 5. Boot Parameters

On a PC the motherboard's BIOS together with the SCSI BIOS provided by most SCSI host adapters takes care of the problem of loading the boot loader's image from a SCSI disk into memory and executing it. This may require some settings to be changed in the motherboard's BIOS. When more than one SCSI adapter is involved, the SCSI BIOS settings may need to change to indicate which one contains the disk with the boot image. The boot image make also come from an ATA (IDE) disk, a bootable CD-ROM or a floppy.

Both lilo and grub are commonly used boot loaders with Linux. Their configuration files are in /etc/lilo.conf and /etc/grub.conf [1] respectively. One difference is that after changing lilo's configuration the lilo command must be executed for the changes to take effect (and there is no equivalent requirement for grub). See their "man" pages for usage information. An excellent paper on lilo and the Linux bootup sequence can be found ftp://icaftp.epfl.ch/pub/people/almesber/booting/bootinglinux-0.ps.gz. For further information on grub see www.gnu.org/software/grub.

Some boot parameters related to the SCSI subsystem:


 single         [enter single user mode]
 <n>            [enter run level <n> {0..6}]
 root=/dev/sda6 [*]
 root=/dev/scsi/host0/bus0/target0/lun0/part6 [*]
 root=/dev/sd/c0b0t0u0p6   [*]
 devfs=mount    [overrides CONFIG_DEVFS_MOUNT=n]
 devfs=nomount  [overrides CONFIG_DEVFS_MOUNT=y]
 init=<command> [executes <command> rather than init]
 quiet          [reduce output to console during boot]
 debug          [increase output to console during boot]
 nmi_watchdog=0 [turn off NMI watchdog on a SMP machine]
 max_scsi_luns=1  [limits SCSI bus scans to lun==0]
 scsi_allow_ghost_devices=<n>

* When devfs is in use the initial read-only mount of the root partition can be done via the old /dev/sd<a><n> notation or the new devfs notation (and two of these are shown). The joint "root=/dev/sda6 single" may be useful when disk or adapter changes have broken the kernel boot load.

The "root=" argument may also be a hex number. For example, if the root partition is on /dev/sda3 then "root=803" is appropriate. The last two digits are the minor device number discussed in an earlier section.

The default argument to the "init" parameter is /sbin/init (see man (8) init). If files such as /etc/fstab have incorrect entries, it may be useful to drop directly into a shell with "init=/bin/bash". However if shared libraries files or their paths are inappropriate this may also fail. That leaves "init=/sbin/sash" which is a statically linked shell with many useful commands (for repairing a system) built in (see man (8) sash).

When Linux fails to boot after reporting a message like:


  VFS: Cannot open root device 08:02

then the kernel expected to find a root partition on device /dev/sda2 and did not. The numbers in the error message are major and minor device numbers (in hex) [see Section 3.2 for the mapping to device names]. In such situations the "root" boot option can be useful (also the rdev command can be used to modify where the boot image looks for the root partition).

Lilo's configuration file /etc/lilo.conf can take the "root=" option in two ways. The normal way is a line like: 'root=/dev/sda2'. In this case /dev/sda2 is converted into major and minor numbers based on the state of the system when the lilo command is executed. This can be a nuisance, especially if hardware is going to be re-arranged. The other way is a line of the form: 'append="root=/dev/sda2"' In this case the /dev/sda2 is passed through to the kernel the next time it is started. This is the same as giving the "root=/dev/sda2" string at the kernel boot time prompt. It is interpreted by the kernel at startup (once the HBAs and their attached devices have been recognized) and thus is more flexible.

Notes

[1]

One slight wrinkle with grub is that /etc/grub.conf is a symbolic link to /boot/grub/grub.conf. This can be useful to know when /boot is a separate partition.