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HOWTO: Convert from single disk to RAID 1 without reinstalling
30/12/08
02:59:11 pm, by richard 
, 2695 words 
Categories: HOWTOs, Ubutnu Linux, Installation and Configuration, OS and Systems Administration
HOWTO: Convert from single disk to RAID 1 without reinstalling
You should see the new menu, the one that talks about RAID 
Select the first menu option and if all went well your system will come up using the RAID1 array. You will also see that you have a file called /this_is_mirror. If you have a file called /this_is_sda1 then you have done something wrong. You missed a step or didn't do the steps in the right order.
Once you are sure you have the RAID array working, and all is well. You need to delete the old partitions on /dev/sda and make them the same as they are on /dev/sdb. You can copy the partition info from the new disk to the old disk. You will need to umount /dev/sda1 if you have remounted it after the reboot.
Use fdisk to delete the old partitions before copying the new partition table over. Remember it is /dev/sda that you need to do the deletions on.
Disk /dev/sda: 4294 MB, 4294967296 bytes
255 heads, 63 sectors/track, 522 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk identifier: 0x000df6a9
Device Boot Start End Blocks Id System
/dev/sda1 * 1 486 3903763+ 83 Linux
/dev/sda2 487 522 289170 82 Linux swap / Solaris
Command (m for help): d
Partition number (1-4): 2
Command (m for help): d
Partition number (1-4): 1
Command (m for help): p
Disk /dev/sda: 4294 MB, 4294967296 bytes
255 heads, 63 sectors/track, 522 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk identifier: 0x000df6a9
Device Boot Start End Blocks Id System
You may need to reboot here depending on what you have mounted. Once the system comes back up you can copy the partition table over.
sfdisk -d /dev/sdb | sfdisk /dev/sda
exit
OK
Disk /dev/sda: 522 cylinders, 255 heads, 63 sectors/track
Old situation:
Units = cylinders of 8225280 bytes, blocks of 1024 bytes, counting from 0
Device Boot Start End #cyls #blocks Id System
/dev/sda1 0 - 0 0 0 Empty
/dev/sda2 0 - 0 0 0 Empty
/dev/sda3 0 - 0 0 0 Empty
/dev/sda4 0 - 0 0 0 Empty
New situation:
Units = sectors of 512 bytes, counting from 0
Device Boot Start End #sectors Id System
/dev/sda1 63 8209214 8209152 fd Linux RAID autodetect
/dev/sda2 8209215 8385929 176715 fd Linux RAID autodetect
/dev/sda3 0 - 0 0 Empty
/dev/sda4 0 - 0 0 Empty
Warning: no primary partition is marked bootable (active)
This does not matter for LILO, but the DOS MBR will not boot this disk.
Successfully wrote the new partition table
Re-reading the partition table ...
If you created or changed a DOS partition, /dev/foo7, say, then use dd(1)
to zero the first 512 bytes: dd if=/dev/zero of=/dev/foo7 bs=512 count=1
(See fdisk(8).)
add the drive /dev/sda to both of the arrays (md0 & md1) and allow them to to sync. You can watch this process with the command in a second terminal. Do not reboot until the resync has finished for both arrays and you see [UU] in the output from /proc/mdstat for both.
sudo mdadm --add /dev/md1 /dev/sda2
md1 : active raid1 sda2[2] sdb2[1]
88256 blocks [2/1] [_U]
resync=DELAYED
md0 : active raid1 sda1[2] sdb1[1]
4104512 blocks [2/1] [_U]
[===>.................] recovery = 19.6% (806976/4104512) finish=1.4min speed=36680K/sec
unused devices: <none>
On a real system this can take hours ~1 hour/250Gb. On VMware with a tiny disk it takes a few minutes. Time for another coffee ;o
We now have a working RAID 1 system where if one disk fails we can still carry on working. We need to make it so we can still boot from either drive when the other has failed. This is repeating some of what we have already setup but it means you can see it is done.
We will be setting the MBR:
on the first hard disk /dev/sda or in grub terms (hd0), to look in the first partition on the first drive for the /boot directory. (hd0,0) in grub speak.
and on the second hard disk /dev/sdb or in grub terms (hd1), to look in the first partition on the second drive for the /boot directory, which is (hd1,0) for grub. Look at the output below for the setup commands.
The commands I used were
root (hd0,0)
setup (hd0)
root (hd1,0)
setup (hd1)
quit
grub> setup (hd0)
Checking if "/boot/grub/stage1" exists... yes
Checking if "/boot/grub/stage2" exists... yes
Checking if "/boot/grub/reiserfs_stage1_5" exists... yes
Running "embed /boot/grub/reiserfs_stage1_5 (hd0)"... 20 sectors are embedded.
succeeded
Running "install /boot/grub/stage1 (hd0) (hd0)1+20 p (hd0,0)/boot/grub/stage2 /boot/grub/menu.lst"
... succeeded
Done.
grub> root (hd1,0)
grub> setup (hd1)
Checking if "/boot/grub/stage1" exists... yes
Checking if "/boot/grub/stage2" exists... yes
Checking if "/boot/grub/reiserfs_stage1_5" exists... yes
Running "embed /boot/grub/reiserfs_stage1_5 (hd1)"... 20 sectors are embedded.
succeeded
Running "install /boot/grub/stage1 (hd1) (hd1)1+20 p (hd1,0)/boot/grub/stage2 /boot/grub/menu.lst"
... succeeded
Done.
Do not forget to update the BIOS, so that it will try one of the two disks and if that fails try booting from the other disk.
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