Bootable Raid

I'm not going to cover the fundamentals of setting up raid0/1/5 on Linux, that is covered in detail elsewhere. The problem I will address is setting up raid on root and making it bootable with standard LILO. The documentation that comes with the LILO sources (not the man pages) and with the raidtools-0.90, covers the details of booting and boot parameters as well as general raid setup - respectively.

There are two scenarios which are covered here. Set up of bootable root raid and the conversion of an existing non-raid system to bootable root raid without data loss.

RAID 1 with standard LILO

To make the boot information redundant and easy to maintain, set up a small RAID1 and mount it on the /boot directory of your system disk. LILO does not know about device 0x9?? and can not find the information at boot time because the raid sub system is not active then. As a simple work around, you can pass LILO the geometry information of the drive(s) and from that, LILO can determine the position of the information needed to load the kernel even though it is on the RAID1 partition. This is because the RAID1 partition is the same as a standard partition but with a raid super-block written at the end. The boot raid set should fall with the first 1024 mbytes of the disk drive. In theory the start of the raid partition could fall anywhere in the 1024 megs, but in practice I was unable to get it to work unless the boot-raid started at the first block of the set. This is probably because of something dumb that I did, but it was not worth following up at the time. Since then I've simply set up all my systems with the boot-raid set as the first partition. I have root raid system configurations with bootable RAID1 mounted on /boot with root raid sets as follows: RAID1, RAID5, RAID10 & RAID1-10 ( 1 mirror + 1 raid0 set). The last has a very peculiar lilo file pair since none of the disk geometries are the same, however, the principals are the same for the initial boot process. The RAID10 and RAID1-10 root mounts require the use of initrd to mount root after the boot process has taken place. See the appendices for the configuration files for all of these example systems.

A conventional LILO config file stripped down looks like this: 

# lilo.conf - assumes drive less than 1024
       boot = /dev/hda
       delay = 40               # extra, but nice
       vga = normal             # not normally needed
       image = /bzImage
       root = /dev/hda1
       read-only
       label = Linux

A raid LILO config file pair would look like this: 

# lilo.conf.hda - primary ide master
       disk=/dev/md0
       bios=0x80
       sectors=63
       heads=16
       cylinders=39770
       partition=/dev/md1
       start=63
       boot=/dev/hda
       map=/boot/map
       install=/boot/boot.b
       image=/boot/bzImage
       root=/dev/md0
       read-only
       label=LinuxRaid

# ---------------------

# lilo.conf.hdc - secondary ide master
       disk=/dev/md0
       bios=0x80                # see note below
       sectors=63
       heads=16
       cylinders=39770
       partition=/dev/md1
       start=63
       boot=/dev/hdc            # this is the other disk
       map=/boot/map
       install=/boot/boot.b
       image=/boot/bzImage
       root=/dev/md0
       read-only
       label=LinuxRaid

# BIOS=line -- if your bios is smart enough (most are not) to detect that that the first disk is missing or failed and will automatically boot from the second disk, then bios=81 would be the appropriate entry here. This is more common with SCSI bios than IDE bios. I simply plan on relocating the drive so it will replace the dead drive C: in the event of failure of the primary boot drive.

The geometry information for the drive can be obtained from fdisk with the command: 

fdisk -ul (little L)
fdisk -ul /dev/hda

Disk /dev/hda: 16 heads, 63 sectors, 39770 cylinders
Units = sectors of 1 * 512 bytes

  Device Boot    Start       End    Blocks   Id  System
/dev/hda1            63     33263     16600+  fd  Linux raid autodetect
/dev/hda2         33264    443519    205128   82  Linux swap
/dev/hda3        443520  40088159  19822320   fd  Linux raid autodetect

* note the listing of the START of each partition

explaination of lilo.conf for raid boot

The raid lilo.conf file above, commented in detail for each entry. 

# lilo.conf.hda - primary ide master
#       the location of the /boot directory that will be
#       designated below as containing the kernel, map, etc...
#       note that this is NOT the actual partition containing
#       the boot image and info, but rather the device
#       that logically contains this directory.
#       in this example, /dev/md1 is mounted on /dev/md0/boot
    disk=/dev/md0

#       tell LILO which bios device to use for boot, i.e. C: drive
    bios=0x80

#       tell LILO the geometry of the device
#       this is usually but not always the "logical"
#       geometry. Check the /proc file system or watch
#       the boot messages when the kernel probes for the drive
#      
    sectors=63
    heads=16
    cylinders=39770

#       this is a dummy entry to make LILO happy so it
#       will recognize the raid set 0x9?? and then find
#       the START of the boot sector. To really see
#       what this was for, read the documentation
#       that comes with the LILO source distribution.
#       This parameter "must" be different than the
#       disk=  entry above. It can be any other mdx
#       device, used or unused and need not be the one
#       that contains the /boot information
#      
    partition=/dev/md1

#       the first sector of the partition containing /boot information
    start=63

#       the real device that LILO will write the boot information to
    boot=/dev/hda

#       logically where LILO will put the boot information
    map=/boot/map
    install=/boot/boot.b

#       logically where lilo will find the kernel image
    image=/boot/bzImage

#       standard stuff after this
#       root may be a raid1/4/5 device
    root=/dev/md0
    read-only
    label=LinuxRaid

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