Hack 63 Migrate to Software RAID

Move your entire root partition to RAID 1 or RAID 5 without a backup and restore.

Software RAID can ensure failover protection even on a low budget. If you want to migrate a system to software RAID, you might be faced with the prospect of doing a complete backup and restore. If you have a low budget, you might not have a spare drive to temporarily back up your data to in addition to the drives you are using for the RAID. With a Knoppix disc and the following instructions, you can migrate a system to RAID 1 or 5 with just the disks you are planning to use for the RAID.

RAID 1 and 5 provide failover, so you can run a system and access files even if a drive in the array has failed. You can leverage failover in RAID 1 and 5 to migrate a partition that is not yet software RAID to RAID 1 or 5 if you create the RAID with a failed drive (the current root partition), copy all of the data over to the newly created RAID, boot onto that RAID, and then add the root partition to the RAID. For this to work, all RAID utilities, such as mkraid and raidhotadd, should already be present on the system. Distribution package names vary, but two examples of package names are raidtools and raidtools2 under Debian. The kernel should already have support for software RAID compiled in, so check the "Multi-device support" section of your kernel's configuration and make sure all the RAID types you wish to use are enabled. This hack covers migrating a complete root filesystem that is on /dev/hda1 to a RAID 1 spanning /dev/hda1 and /dev/hdb1, or a RAID 5 spanning /dev/hda1, /dev/hdb1, and /dev/hdc1.

First create the array; create /etc/raidtab in Knoppix and add the following configuration for a RAID 1 array:

raiddev /dev/md0

        raid-level      1

        nr-raid-disks   2

        nr-spare-disks  0

        chunk-size      4

        persistent-superblock 1

        device          /dev/hdb1

        raid-disk       0

        device          /dev/hda1

        failed-disk     1

Or add the following configuration for a RAID 5 array:

raiddev /dev/md0

        raid-level      5

        nr-raid-disks   3

        nr-spare-disks  0

        persistent-superblock 1

        parity-algorithm        left-symmetric

        chunk-size      32

        device          /dev/hdb1

        raid-disk       0

        device          /dev/hdc1

        raid-disk       1

        device          /dev/hda1

        failed-disk       2

Notice that the main root partition, /dev/hda1, is listed as a failed-disk for the moment. The RAID tools have problems starting the RAID when the first disk is a failed disk, so list /dev/hda1 last. Once you create this file, start the RAID with this command:

knoppix@tty0[knoppix]$ sudo mkraid /dev/md0

If you check /proc/mdstat, it lists the RAID (in this example, a RAID 1):

knoppix@tty0[knoppix]$ cat /proc/mdstat

Personalities : [raid1]

read_ahead 1024 sectors

md0 : active raid1 hdb1[1]

      2621312 blocks [2/1] [U_]



unused devices:

Now the RAID is created, and you can format it with the filesystem of your choice with:

knoppix@tty0[knoppix]$ sudo mkfs -t xfs /dev/md0

Since /dev/hda1 is marked failed, the RAID does not overwrite all of the files on that partition yet, and you are safe to format the new RAID partition and copy files to it.

Next, you must create a temporary directory to mount the new RAID, mount the RAID and the original partition read/write, and then copy the system over to the RAID using the same method covered in [Hack #61] :

knoppix@tty0[knoppix]$ sudo mkdir /mnt/temp

knoppix@tty0[knoppix]$ sudo mount -o rw /dev/md0 /mnt/temp

knoppix@tty0[knoppix]$ sudo mount /dev/hda1 /mnt/hda1



knoppix@tty0[knoppix]$ cd /mnt/hda1



knoppix@tty0[hda1]$ sudo sh -c "find . -xdev -print0 | cpio -pa0V /mnt/temp"

Once the copy process finishes, copy the /etc/raidtab in Knoppix to /mnt/temp/etc/. Then edit /mnt/temp/etc/fstab and make sure the entry for /dev/hda1 is changed to /dev/md0. You must also edit lilo or grub configuration files, and make sure that any root device configuration now references /dev/md0 instead of /dev/hda1. Be sure to leave any boot device configuration alone so that it installs the boot code onto /dev/hda instead of /dev/md0. If you use lilo as your boot loader, you must also run lilo to update the MBR as covered in [Hack #52] .

Once you update all of the configuration files, add the primary partition to the RAID by unmounting /dev/hda1:

knoppix@tty0[knoppix]$ sudo umount /dev/hda1

Note that the data on the partition has remained untouched. Once you add this drive to the array, it will be overwritten with whatever files are already in /dev/md0, so be sure you are ready before continuing.

To add the drive to the array, edit /etc/raidtab on the ramdisk, not the RAID, and replace failed-disk with raid-disk. Then, add the drive to the array with the following command:

knoppix@tty0[knoppix]$ sudo raidhotadd /dev/md0 /dev/hda1

Run the following command to monitor the RAID as it updates /dev/hda1 with all of the mirroring information:

knoppix@tty0[knoppix]$ watch cat /proc/mdstat

You can still use and write to the RAID while this is going on, so copy the new /etc/raidtab to /mnt/temp/etc/. You must wait for the mirror to be completely synced before moving to the next step (/proc/mdstat lets you know when you are in sync).

Now enable RAID autodetection for both of the partitions; unmount and stop the RAID:

knoppix@tty0[knoppix]$ sudo umount /dev/md0

knoppix@tty0[knoppix]$ sudo raidstop /dev/md0

Then run fdisk or cfdisk, and change the partition type for both /dev/hda1 and /dev/hdb1 from 83 to fd. Then write the changes to make sure that the Linux kernel autodetects this RAID as it boots (which is important because it is the root partition). After autodetection has been enabled, reboot into your new software RAID root partition.

6.13.1 See Also

• The Software RAID HOWTO: /usr/share/doc/raidtools2/Software-RAID.HOWTO on your Knoppix disc.

• The raidhotadd, mkraid, raidstop, and other RAID tools manpages (type man commandname in a console).