Article

Overview
Installation Details
Did it Work?
Related Links
Conclusion

Overview

The wise administrator builds redundancy into the systems he manages. Using a redundant array of independent disks (RAID) is one way to build redundancy. A redundancy strategy is not a backup strategy. I would like to be clear that this article is not a replacement for a properly implemented backup strategy. I personally believe the best RAID solution is hardware RAID. However, when budgets are constrained, software RAID is an alternative. This article focuses on the process of installing the operating system onto a software RAID1 mirror.

The scenario is based on SUSE Linux Enterprise Server 10 Service Pack 1, and includes the base and Web and Lamp patterns only. The server should have two disks of the same size, 2Gb in this case. This is more for clarity than an actual requirement. Then you will create two partitions of type Linux RAID on each of the disks. The partitions need to be the same size on each disk. You will then create the software RAID device using the "Linux RAID" partitions you created earlier. Finally, you will format each software RAID device; one with swap, the other with a root file system. The scenario has been tested on the i386 and x86_64 architectures.

Installation Details

1. Start the SLES10 SP1 install as you usually do, until you get to the Installation Settings screen, then click Change, Partitions.


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Figure 1 - Installation settings

2. Click Create Custom Partition Setup and Next.


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Figure 2 - Create custom partition setup

3. Select Custom Partitioning and Next


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Figure 3 - Custom partitioner

4. Create a 500MB Linux RAID (type 0xFD) partition for swap, and use the rest of the space for a Linux RAID (type 0xFD) partition for root.


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Figure 4 - Creating Linux RAID partition for swap

5. Create the corresponding partitions on the other disk.
6. When you have finished creating all the Linux RAID partitions, your Expert Partitioner should look like Figure 5 below. Notice there is a partition of equal size on each disk that serves as a place holder for swap and root. Now you will assemble the mirror using each of these double partitions.

   Select RAID and Create RAID.



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Figure 5 - Linux RAID partitions created

7. Select RAID 1 (Mirroring), and Next.


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Figure 6 - Creating swap mirror

8. The current RAID device should be /dev/md0. Add both 500MB Linux RAID partitions to the md0 RAID.


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Figure 7 - Assembling swap mirror

9. Once you have added md0 to both 500Mb partitions, click Next.


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Figure 8 - Assembled swap mirror

10. Format the /dev/md0 device with a swap file system, and click Finish.


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Figure 9 - Formatting swap mirror

11. Now repeat the previous four steps for the root partition. First, select RAID and Create RAID.


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Figure 10 - swap mirror created

12. Select RAID 1 (Mirroring) and Next.


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Figure 11 - Creating root mirror device

13. The current RAID device should be /dev/md1. Add both remaining Linux RAID partitions to the md1 RAID, and click Next.


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Figure 12 - Assembled root mirror device

14. Format the /dev/md1 device with a reiser file system, mounted on /; and click Finish.


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Figure 13 - Formatting root MD device

15. When your done, the partitioner screen should look something like this:


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Figure 14 - Completed expert partitioner screen

I created only two system partitions with a Linux RAID partition mirrored for each. If you have more system partitions, like /var, then you will need to have a pair of Linux RAID partitions and a /dev/md? device for each additional system partition.

16. Click Finish.
17. Notice that the boot loader section references the /dev/md? devices.


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Figure 15 - Observe boot configuration

18. Complete the installation as usual.
19. Once the installation is complete, you need to finish the GRUB install. Since GRUB does not understand MD devices, it is only installed on the first disk. I like to make sure it is installed the same way on both disks.
20. Login as root, and type "grub". Follow the steps in Figure 16 below.


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Figure 16 - GRUB install steps

Did it Work?

This section is intended to show what the system should look like using various commands after a successful install. Troubleshooting failed installs or partial installs is outside the scope of this article.

• Check the /etc/fstab file to ensure the software RAID devices are used to mount root and swap.

raid1:~ # cat /etc/fstab
/dev/md1  /                  reiserfs  acl,user_xattr    1 1
/dev/md0  swap               swap      defaults          0 0
proc      /proc              proc      defaults          0 0
sysfs     /sys               sysfs     noauto            0 0
debugfs   /sys/kernel/debug  debugfs   noauto            0 0
devpts    /dev/pts           devpts    mode=0620,gid=5   0 0
/dev/fd0  /media/floppy      auto      noauto,user,sync  0 0

• The /proc/mdstat shows the current status of the array. I will also show you the current state of a sync in progress. It is a brief version of the mdadm --detail output.

raid1:~ # cat /proc/mdstat
Personalities : [raid1] [raid0] [raid5] [raid4] [linear]
md1 : active raid1 sda2[0] sdb2[1]
      1582336 blocks [2/2] [UU]

md0 : active(auto-read-only) raid1 sda1[0] sdb1[1]
      513984 blocks [2/2] [UU]

unused devices: <none>

• The /proc/cmdline shows the kernel command line configured at boot time. The source for these parameters is the /boot/grub/menu.lst and whatever is typed on the "Boot Options" line in the GRUB menu at boot time. The value in this case is to see that the root= parameter is pointing to the mirrored RAID array.

raid1:~ # cat /proc/cmdline
root=/dev/md1 vga=0x332 resume=/dev/md0 splash=silent showopts

• The mdadm command provides the most complete view of the software RAID array. The state should be "clean" and "active sync" for both devices in the mirror.

raid1:~ # mdadm --detail /dev/md1
/dev/md1:
        Version : 00.90.03
  Creation Time : Tue Mar  4 01:22:48 2008
     Raid Level : raid1
     Array Size : 1582336 (1545.51 MiB 1620.31 MB)
  Used Dev Size : 1582336 (1545.51 MiB 1620.31 MB)
   Raid Devices : 2
  Total Devices : 2
Preferred Minor : 1
    Persistence : Superblock is persistent

    Update Time : Thu Mar  6 18:56:46 2008
          State : clean
 Active Devices : 2
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 0

           UUID : 3858d3c1:b6e37b6d:8e2de91f:260fb55b
         Events : 0.5205

 Number Major Minor RaidDevice State
    0     8      2      0      active sync  /dev/sda2
    1     8     18      1      active sync  /dev/sdb2

Related Links

• Understanding how RAIDed Disks Interact with the SLES Boot Process
• Software RAID: Beyond YAST
• Migrating SLES to Software RAID1

Conclusion

Installing the SLES operating system is rather straight forward. The only real catch is to make sure you install GRUB onto both disks when the install is complete. The mirrored array provides redundancy for the system disk. If one of the disks becomes damaged or fails, then you can recover quickly using the other mirrored disk. However, always make sure you have a current working backup.