Creating a software raid using mdadm

Gentoo: emerge mdadm && rc-update add mdadm default && /etc/init.d/mdadm start
Debian/Ubuntu: apt-get install mdadm
RedHat/Fedora: yum install mdadm

You can activate the kernel module by executing: modprobe raid1

First of all, we are going to create some nodes:
mknod /dev/md1 b 9 1
mknod /dev/md2 b 9 2
mknod /dev/md3 b 9 3
mknod /dev/md4 b 9 4

Then you can create your new raid devices:
mdadm –create –verbose /dev/md1 –level=1 –raid-devices=2 /dev/hda1 /dev/hdb1
mdadm –create –verbose /dev/md2 –level=1 –raid-devices=2 /dev/hda2 /dev/hdb2
mdadm –create –verbose /dev/md3 –level=1 –raid-devices=2 /dev/hda3 /dev/hdb3
mdadm –create –verbose /dev/md4 –level=1 –raid-devices=2 /dev/hda4 /dev/hdb4

After the creation, the drives are synchronised. This may take some time. You can watch the status using
watch cat /proc/mdstat

When the synchronisation has finished you can format the new partitions:
mke2fs -j /dev/md1
mke2fs -j /dev/md2
mke2fs -j /dev/md3
mke2fs -j /dev/md4

You should save your new raid configuration using the following command:
mdadm –detail –scan >> /etc/mdadm.conf
(the path may vary, depending on the underlying system it may also be /etc/mdadm/mdadm.conf)
Now you should add your new devices to /etc/fstab.

Now you can partition (fdisk) and mount your drives and you have your raid running!