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<TITLE>Boot + Root + Raid + Lilo : Software Raid mini-HOWTO: Bootable Raid</TITLE>
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<H2><A NAME="s3">3. Bootable Raid</A></H2>
<P>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 <B>standard</B> 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.
<P>
<P>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.
<P>
<H2><A NAME="ss3.1">3.1 Booting RAID 1 with standard LILO</A>
</H2>
<P>To make the boot information redundant and easy to maintain, set up a small
RAID1 and mount it on the <B>/boot</B> 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 <B>/boot</B> 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
<I>initrd</I> to mount root after the boot process has taken place.
See the appendices for the configuration files for all of these example
systems.
<P>
<P>A conventional LILO config file stripped down looks like this:
<P>
<PRE>
# 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
</PRE>
<P>
<P>A raid LILO config file pair would look like this:
<P>
<PRE>
# 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
</PRE>
<P># 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 <B>bios=81</B> 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.
<P>
<P>The geometry information for the drive can be obtained from fdisk with the
command:
<P>
<PRE>
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
</PRE>
<P>
<H2><A NAME="ss3.2">3.2 Detailed explaination of lilo.conf for raid boot</A>
</H2>
<P>The raid lilo.conf file above, commented in detail for each entry.
<P>
<PRE>
# 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
</PRE>
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