Convert RedHat to Raid
The technote details how to convert a linux system with non RAID devices to run with a Software RAID configuration.
This scenario was tested with Redhat 7.1, but should be applicable to any release which supports Software RAID (md) devices.
Pre-conversion example system
The test system contains two SCSI disks, sda and sdb both of of which are the same physical size. As part of the test setup, I configured both disks to have the same partition layout, using fdisk to ensure the number of blocks for each partition was identical.
DEVICE MOUNTPOINT SIZE DEVICE MOUNTPOINT SIZE /dev/sda1 / 2048MB /dev/sdb1 2048MB /dev/sda2 /boot 80MB /dev/sdb2 80MB /dev/sda3 /var/ 100MB /dev/sdb3 100MB /dev/sda4 SWAP 1024MB /dev/sdb4 SWAP 1024MB
In our basic example, we are going to set up a simple RAID-1 Mirror, which requires only two physical disks.
Step-1 - boot rescue cd/floppy
The redhat installation CD provides a rescue mode which boots into linux from the CD and mounts any filesystems it can find on your disks.
At the lilo prompt type
lilo: linux rescue
With the setup described above, the installer may ask you which disk your root filesystem in on, either sda or sdb. Select sda.
The installer will mount your filesytems in the following way.
DEVICE MOUNTPOINT TEMPORARY MOUNT POINT /dev/sda1 / /mnt/sysimage /dev/sda2 /boot /mnt/sysimage/boot /dev/sda3 /var /mnt/sysimage/var /dev/sda6 /home /mnt/sysimage/home
Note: - Please bear in mind other distributions may mount your filesystems on different mount points, or may require you to mount them by hand.
Step-2 - create a /etc/raidtab file
Create the file /mnt/sysimage/etc/raidtab (or wherever your real /etc file system has been mounted.
For our test system, the raidtab file would like like this.
raiddev /dev/md0 raid-level 1 nr-raid-disks 2 nr-spare-disks 0 chunk-size 4 persistent-superblock 1 device /dev/sda1 raid-disk 0 device /dev/sdb1 raid-disk 1
raiddev /dev/md1 raid-level 1 nr-raid-disks 2 nr-spare-disks 0 chunk-size 4 persistent-superblock 1 device /dev/sda2 raid-disk 0 device /dev/sdb2 raid-disk 1
raiddev /dev/md2 raid-level 1 nr-raid-disks 2 nr-spare-disks 0 chunk-size 4 persistent-superblock 1 device /dev/sda3 raid-disk 0 device /dev/sdb3 raid-disk 1
Note: - It is important that the devices are in the correct order. ie. that /dev/sda1 is raid-disk 0 and not raid-disk 1. This instructs the md driver to sync from /dev/sda1, if it were the other way around it would sync from /dev/sdb1 which would destroy your filesystem.
Now copy the raidtab file from your real root filesystem to the current root filesystem.
(rescue)# cp /mnt/sysimage/etc/raidtab /etc/raidtab
Step-3 - create the md devices
There are two ways to do this, copy the device files from /mnt/sysimage/dev or use mknod to create them. The md device, is a (b)lock device with major number 9.
(rescue)# mknod /dev/md0 b 9 0 (rescue)# mknod /dev/md1 b 9 1 (rescue)# mknod /dev/md2 b 9 2
Step-4 - unmount filesystems
In order to start the raid devices, and sync the drives, it is necessary to unmount all the temporary filesystems.
(rescue)# umount /mnt/sysimage/var (rescue)# umount /mnt/sysimage/boot (rescue)# umount /mnt/sysimage/proc (rescue)# umount /mnt/sysimage
Please note, you may not be able to umount /mnt/sysimage. This problem can be caused by the rescue system - if you choose to manually mount your filesystems instead of letting the rescue system do this automat- ically, this problem should go away.
Step-5 - start raid devices
Because there are filesystems on /dev/sda1, /dev/sda2 and /dev/sda3 it is necessary to force the start of the raid device.
(rescue)# mkraid --really-force /dev/md2
You can check the completion progress by cat'ing the /proc/mdstat file. It shows you status of the raid device and percentage left to sync.
Continue with /boot and /
(rescue)# mkraid --really-force /dev/md1 (rescue)# mkraid --really-force /dev/md0
he md driver syncs one device at a time.
Step-6 - remount filesystems
Mount the newly synced filesystems back into the /mnt/sysimage mount points.
(rescue)# mount /dev/md0 /mnt/sysimage (rescue)# mount /dev/md1 /mnt/sysimage/boot (rescue)# mount /dev/md2 /mnt/sysimage/var
Step-7 - change root
You now need to change your current root directory to your real root file system.
(rescue)# chroot /mnt/sysimage
Step-8 - edit config files
You need to configure lilo and /etc/fstab appropriately to boot from and mount the md devices.
Note: - The boot device MUST be a non-raided device. The root device is your new md0 device. eg.
boot=/dev/sda map=/boot/map install=/boot/boot.b prompt timeout=50 message=/boot/message linear default=linux
image=/boot/vmlinuz label=linux read-only root=/dev/md0
/dev/md0 / ext3 defaults 1 1 /dev/md1 /boot ext3 defaults 1 2 /dev/md2 /var ext3 defaults 1 2 /dev/sda4 swap swap defaults 0 0
Step-9 - run LILO
With the /etc/lilo.conf edited to reflect the new root=/dev/md0 and with /dev/md1 mounted as /boot, we can now run /sbin/lilo -v on the chrooted filesystem.
Step-10 - change partition types
The partition types of the all the partitions on ALL Drives which are used by the md driver must be changed to type 0xFD.
Use fdisk to change the partition type, using option 't'.
(rescue)# fdisk /dev/sda (rescue)# fdisk /dev/sdb
Use the 'w' option after changing all the required partitions to save the partion table to disk.
Step-11 - resize filesystem
When we created the raid device, the physical partion became slightly smaller because a second superblock is stored at the end of the partition. If you reboot the system now, the reboot will fail with an error indicating the superblock is corrupt.
Resize them prior to the reboot, ensure that the all md based filesystems are unmounted except root, and remount root read-only.
(rescue)# mount / -o remount,ro
You will be required to fsck each of the md devices. This is the reason for remounting root read-only. The -f flag is required to force fsck to check a clean filesystem.
(rescue)# e2fsck -f /dev/md0
This will generate the same error about inconsistent sizes and possibly corrupted superblock.Say N to 'Abort?'.
(rescue)# resize2fs /dev/md0
Repeat for all /dev/md devices.
Step-12 - checklist
The next step is to reboot the system, prior to doing this run through the checklist below and ensure all tasks have been completed.
- All devices have finished syncing. Check /proc/mdstat
- /etc/fstab has been edited to reflect the changes to the device names.
- /etc/lilo.conf has beeb edited to reflect root device change.
- /sbin/lilo has been run to update the boot loader.
- The kernel has both SCSI and RAID(MD) drivers built into the kernel.
- The partition types of all partitions on disks that are part of an md device have been changed to 0xfd.
- The filesystems have been fsck'd and resize2fs'd.
Step-13 - reboot
You can now safely reboot the system, when the system comes up it will auto discover the md devices (based on the partition types).
Your root filesystem will now be mirrored.