Lets create the raid:

mdadm --create /dev/md0 --level=5 --raid-devices=2 /dev/sdb /dev/sdc

The raid gets created! and we can monitor it with

cat /proc/mdstat

When it has finished intitialising, create a file system on the raid array (ext3):

mke2fs -j /dev/md0

create a mount point (/raid) and mount it

mkdir /raid

mount /dev/md0 /raid

df then reports it as having 2Gb free.  Both my VM drives sdb and sdc are 2Gb, so the assumption is it is simply mirroring the data in 2 drive mode.  This is exactly what we want, as when I get a new drive later on, I want to add it to the raid and see an increase in disk space.  So lets test that, shutdown my machine and add a new drive.

Add the drive to the array

mdadm --manage --add /dev/md0 /dev/sdd

now when I run cat /proc/mdstat it says there are 3 drives in the arras but sdd is marked (S)

Lets now grow the array

mdadm --grow --raid-disk=3 /dev/md0

Watch the progress with cat /proc/mdstat and when complete we can mount it. (adding the 2Gb took about 5 mniutes! eeek! to grow the array).

After completion /proc/mdstat now reports 4gb available, but the file system on the raid still thinks it’s 2gb.

So let’s resize it:

e2fsck -f /dev/md0

resize2fs /dev/md0

Re mount /dev/md0 and df now reports 4Gb.

Success.