Increase Performance, Reliability and Capacity with Software RAID

Linux software RAID provides a flexible software alternative to hardware RAID with excellent performance.
Managing Arrays

In addition to creating and rebuilding arrays, you need to be familiar with a few other tasks. It is important to understand how to start and stop arrays. Run the following commands to unmount and stop the RAID 1 array you created earlier:

# umount /dev/md0
# mdadm -S /dev/md0
# cat /proc/mdstat

As you can see, the array no longer is listed in /proc/mdstat. In order to start your array again, you need to assemble it (there isn't a start command). Run the following commands to assemble and remount your array:

# mdadm -As /dev/md0
# mount /dev/md0
# cat /proc/mdstat

Sometimes it's useful to place an array in read-only mode. Before you do this, you need to unmount the filesystem (you can't just remount as read-only). If you try to place an array in read-only mode while it is mounted, mdadm will complain that the device is busy:

# umount /dev/md0
# mdadm -o /dev/md0
# cat /proc/mdstat
# mount /dev/md0

Notice how /dev/md0 is now read-only, and the filesystem was mounted as read-only automatically. Run the following commands to change the array and filesystem back to read-write mode:

# mdadm -w /dev/md0
# mount -o remount,rw /dev/md0 

mdadm can be configured to send e-mail notifications regarding the status of your RAID arrays. Ubuntu automatically starts mdadm in monitoring mode for you; however, it currently is configured to send e-mail to the root user on the local system. To change this, edit /etc/mdadm/mdadm.conf and set MAILADDR to your e-mail address, then restart the mdadm dæmon:

# vim /etc/mdadm/mdadm.conf

Set MAILADDR to <your e-mail address>, and then do:

# /etc/init.d/mdadm restart

Run the following command to test your e-mail notification setup:

# mdadm --monitor --scan -t -1

Converting a Server to RAID 1

If you are building a new server, you can use the Ubuntu Alternate install CD to set up your system on a software RAID array. If you don't have the luxury of performing a clean install, you can use the following process to convert your entire server to a RAID 1 array remotely. This requires your server to have an additional drive that is the same size or larger than the first disk. These instructions also assume you are using the server edition of Ubuntu 8.04 (Hardy), but the process is similar in other Linux distributions. You always should test a procedure like this before performing it on a production server.

Install mdadm and verify that the software RAID kernel module was loaded properly:

# apt-get install mdadm
# cat /proc/mdstat

Copy the partition table from your first drive to your second drive, and set the partition types to “Linux RAID autodetect”:

# sfdisk -d /dev/sda | sfdisk /dev/sdb
# fdisk /dev/sdb
> t
> 1
> fd
> t
> 2
> fd
> w

Create the RAID 1 arrays for the root and swap partitions, and update the mdadm configuration file. This time, specify that the first drive is “missing”, which will delay the rebuild until you add the first drive to the array. You don't want to mess with the first drive until you are sure the RAID configuration is working correctly:

# mdadm -C /dev/md0 -n2 -l1 missing /dev/sdb1  # root
# mdadm -C /dev/md1 -n2 -l1 missing /dev/sdb2  # swap
# cat /proc/mdstat
# mdadm --detail --scan >> /etc/mdadm/mdadm.conf

Modify /boot/grub/menu.lst so your server boots from the array:

# vim /boot/grub/menu.lst


  • Add fallback 1 to a new line after default 0.

  • Change the kopt line to # kopt=root=/dev/md0 ro.

  • Copy the first kernel entry and change (hd0,0) to (hd1,0).

  • Change root=xxx to root=/dev/md0 in the new kernel entry.

When your server is booting up, it needs to be able to load the RAID kernel modules and start your array. Use the following command to update your initrd file:

# update-initramfs -u

At this point, you can create and mount the filesystem, then copy your data to the additional drive. Make sure all of your applications are shut down and the server is idle; otherwise, you run the risk of losing any data modified after you run the rsync command:

# mkswap /dev/md1
# mkfs.ext3 /dev/md0
# mkdir /mnt/md0
# mount /dev/md0 /mnt/md0
# rsync -avx / /mnt/md0

To mount the RAID arrays automatically when your server reboots, modify /mnt/md0/etc/fstab and replace /dev/sda1 with /dev/md0, and replace /dev/sda2 with /dev/md1. You do this only on the second drive, in case you need to fall back to the old setup if something goes wrong:

# vim /mnt/md0/etc/fstab