Software raid on Linux

to create a software raid in Linux we use the mdadm command
first you will need the partitions available to you for the raid, using fdisk you must tag the partitions with the "linux raid autodetect " tag using the hex code id of fd to do this just type fdisk /dev/hda (you use the name of the partion you want to tag then type t and then type fd type w to save and then type partprobe to probe the changes into your kernel

next you need to create your raid partition , lets create a raid 1 partition which is a mirrored set and it uses 2 disks, but we are going to add a 3rd disk and use it as a spare, so if one of the 2 disks in the mirror break the spare will automatically replace it in the raid configuration and rebuild itself.

so we have 3 partitions taged with the "linux raid autodetect" tag id fd

to create a raid 1 partition we type mdadm -C /dev/md0 -l 1 -n 2 -x 3 -a yes /dev/hd{3..5}
-C = create
-l 1 = raid level 1
-n 2 = how many disks we will be using
-x 1= x stands for spare (how many spare disks we will be using
-a yes = instructs udev to create the md device file if it does not already exist
/dev/hd{3..5} are the disks you want to use in the raid you could type them separately ie /dev/hd3 /dev/hd4 /dev/hd5 if you want but /dev/hd{3..5} is shorter to type and is the exact same thing.

once done you need to format your raid partition.
mkfs.ext3 /dev/md0
next create a directory were you want to mount your raid partition
mkdir /media/raid1
lets give the partition a label we will use the label name in the fstab file this is not necessary however it is good practice to label your partitions so that if they ever change their /dev position the label will remain the same and they will still mount.
e2label /dev/md0 raid1

then we can mount it in fstab using its label name ... in your /etc/fstab file add the following line
LABEL=raid1 /media/raid5 ext3 defaults 1 2
save your fstab file and type mount-a to have your fstab file read and loaded by the system
to monitor our raid partition we can type cat /proc/mdstat
or watch -n1 cat /proc/mdstat this will update the monitoring process every second

you can even simulate a drive failure to test if your raid works,
type watch -n1 cat /proc/mdstat in a terminal window and set that window to always be on top , then open another window to fail your drive in and you can watch your raid rebuild itself in the monitoring window.
in the other window type mdadm /dev/md0 -f /dev/hd3 (this will fail drive /dev/hd3 in your raid)
(watch what happens in your monitoring window) you can then remove the failed drive from the raid set by typing mdadm /dev/md0 -r /dev/hd3
and to put a new drive back into you raid set type mdadm /dev/md0 -a /dev/hd3