Andrew Birkett’s blog

# Creating and destroying disks from the safety of your own console
mkdir ~/raid; cd ~/raid
 
# Create two 10Mb files called disk0 and disk1
for d in 0 1; do dd if=/dev/zero of=disk${d} bs=1024 count=10240; done
 
# Make them available as block devices using the loopback device
for d in 0 1; do sudo losetup /dev/loop$d disk$d; done
 
# Combine the two 'disks' into a RAID-1 mirrored block device
# Using '--build' rather than '--create' means there is no device
# specific metadata, and so the contents of the disks will be identical
sudo mdadm --build --verbose /dev/md0 --level=1 --raid-devices=2 /dev/loop[01]
 
# Create a filesystem on our raid device and mount it
sudo mkfs.ext3 /dev/md0 
mkdir /tmp/raidmnt
sudo mount /dev/md0 /tmp/raidmnt
sudo chown $USER /tmp/raidmnt
 
# The contents of both disks change in unison
md5sum disk[01]
date > /tmp/raidmnt/datefile
sync
md5sum disk[01]
 
# If we mark one disk as failed, disk contents diverge
sudo mdadm --fail /dev/md0 /dev/loop0
date > /tmp/raidmnt/datefile
sync
md5sum disk[01]
 
# Remove the failed disk and readd it, and RAID1 will sync
sudo mdadm --remove /dev/md0 /dev/loop0
sudo mdadm --add /dev/md0 /dev/loop0
sleep 1
md5sum disk[01]
 
# Add a third (unused) disk into the system to test failover
dd if=/dev/zero of=disk2 bs=1024 count=10240
sudo losetup /dev/loop2 disk2
sudo mdadm --add /dev/md0 /dev/loop2
sudo mdadm --detail /dev/md0
 
# When one of original two disks fail, the new disk gets used
md5sum disk[012]
sudo mdadm --fail /dev/md0 /dev/loop0
date > /tmp/raidmnt/datefile
sync
md5sum disk[012]
 
# Tidy up the world
sudo umount /dev/md0
sudo mdadm -S /dev/md0
for x in /dev/loop[012]; do sudo losetup -d $x; done
rm -rf /tmp/raidmnt ~/raid