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Partition types:
. primary - one of the 4 partitions limited by the master boot record (mbr); mbr resides in the 1st sector of the disk (ie first 512 bytes).
. extended - one of the primary partitions that is logically broken to create more than 4 partitions.
. logical - one of constituents of the extended partition.
-the boot partition must be a primary partition and reside completely in the first 1024 cylinders;
-this is because the bios can't read or boot from the boot partition, if this condition is not met.
-usually 100 MB for boot partition is ok.
-partition recommendations:
. first define boot
. then define swap
. then define /usr, /opt, /var in a single large partition - perhaps / ?
. after that define rest of the system like /home etc.
-mbr lives in the first sector of the first primary partition. the mbr contains the partition table, info about the partitions in the system.
-Since a sector = 512 bytes, mbr = 512 bytes & in turn partition table = 512 bytes.
-every media (disk, floppy, cd) contains an executable code in the mbr even if the code is only to put a message "Non-bootable disk in drive A:".
-this is the code that is loaded by bios during the bootstrap. this is called 'stage1 boot loader'.
-this code from mbr / stage1 boot loader (ie first sector) looks for active primary partition and loads the first few blocks of that partition into ram.
-these few blocks from active primary partition comprise 'stage 2 boot loader'.
-stage 1 + stage 2 = boot strapping.
-the above works fine if there is only one os in the system. but if there are multiple os, then another piece of code called boot-loader is needed.
-the boot-loader allows the user to select one of the os to boot, ie choose which set of first os-disk-blocks to load into ram.
-Note: even if a system can have 4 primary partitions, it can still have more than 4 bootable os partitions; this is possible bec of boot-loaders.
-eg of boot loaders = grub, lilo, bootmagic.
-bootloader lives in an os partition and is invoked by the mbr. [[ (mbr.exe) ] --> (bootloader.exe) --> (rest of os partition) ]
-Why is grub > lilo?
Because when changes are made to the system (new os, new kernel) lilo boot-setup needs to be recreated from the cmd line whereas for
grub only the grub.conf file needs to be re-edited.
. MBR or
. the partition boot record of a partition or
. on removable media (floppy, cd, usb key)
-lilo config file is /etc/lilo.conf
-Grub can be installed in the
. MBR or
. the partition boot record of a partition or
. on removable media (floppy, cd, usb key)
-grub config file is /boot/grub/grub.conf
-grub cmd /sbin/grub or /usr/sbin/grub is a small but powerful shell that supports several grub cmds.
-grub.conf is generated by anaconda, the linux installer.
-In the grub.conf file :
. all counting in grub.conf starts with 0. eg default=2 => 3rd stanza.
. splashimage = the background image for the grub boot menu.
. root = partition that will be booted (ie /boot partition).
eg: root (hd0, 6) => /dev/hda7 = /boot partition.
root (hd1, 10)=> /dev/hdb11 = /boot partition.
root (hd2, 7) => /dev/hdc8 = /boot partition.
. initrd => initial RAM disk => the disk partition that contains modules needed by kernel before file systems can be mounted.
-To install grub to a removable disk use the 'grub-install' cmd
-eg: for floppy disk:
# grub-install /dev/fd0
-note: this loads the stage 1 boot loader to the first sector of the floppy disk which loads stage2 boot loader (which lives on the hard disk)
-stage1 bootloader on floppy will still show empty when mounted as the first sector does not show up in the filesystem.
-stage1 bootloader only has a list of block addresses for stage2 bootloader.
-So if a partition address changes, grub needs to be reconfigured in order for stage1 to locate stage2 bootloader.
-Anyone having access to the grub cmd line also has access to files on the filesystems without the restrictions of file / owner permissions.
-the habit of creating a boot floppy or usb disk is good because it can help in case the mbr gets overwritten by another os install.
-even if the boot floppy or usb disk are not available, then linux install disk can be used to go in recovery mode and then mbr reinstalled.
-eg: # chroot /mnt/sysimage [ on the recovery window, to make /mnt/sysimage as root mount directory ]
# grub-install [ reinstalls mbr ]
-Remember:
[[ mbr = 1st sector = stage1 boot loader ]]
|-> stage2 bootloader partition 1 -> grub menu option 1
|-> stage2 bootloader partition 2 -> grub menu option 2
|-> stage2 bootl<<<oader partition 3 -> grub menu option 3
. . . . . .
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