Partition used:  (these are small test Partitions) 
/dev/hdc5           14725       14726       16033+  83  Linux
/dev/hdc6           14727       14728       16033+  83  Linux
/dev/hdc7           14729       14730       16033+  83  Linux
/dev/hdc8           14731       14732       16033+  83  Linux
( the real array I built has uses /dev/hdc1 /dev/hdd1 .... and each
partition takes up the entire 160Gb drive. ) 

(create array)
# mdadm --create /dev/md1 --level=6 --raid-devices=4 /dev/hdc5 /dev/hdc6 /dev/hdc7 missing
mdadm: array /dev/md1 started.

"missing" leaves a drive missing. Raid 6 lets me do as many as 2
missing. Raid 5 lets me set one drive missing. I found this useful for
building an array when some of the drives I intended to use are part of
the old array I intend to replace.

what is /dev/md1 ?
    /dev/md# is a device that represents the multiple device drive
     (a logical device composed of many other devices)
    you create one for each raid array you create. I am using /dev/md1 
    because /dev/md0 was already used.


to restart (assemble an existing array ) 

#/sbin/mdadm -A /dev/md1

(this reads /etc/mdadm.conf  to figure our which devices are used to 
create the array )


(check status)
# mdadm --query /dev/md1
/dev/md1: 31.13MiB raid6 4 devices, 0 spares. Use mdadm --detail for more detail.
/dev/md1: No md super block found, not an md component.

The "No md super block found.." message is not an error. 
The raid device itself is not a member of a raid device, so 
it does not have such a block. mdadm will find this block if you
call mdadm on a device within the array.

# mdadm --query /dev/hdc5
/dev/hdc5: is not an md array
/dev/hdc5: device 0 in 4 device inactive raid6 md1.  Use mdadm --examine for more detail.


(check status verbose)
# mdadm --detail /dev/md1

to create file system on raid drive i used:
# mke2fs -b 4096  -R stride=64  -J  size=400 /dev/md1
( this journal is actually to big for this test array, I used 4 here)
-R stride=64 is supposed to get the block size to work out evenly with
the raid array.

to "hot add" a drive to the Array.
# mdadm /dev/md1 -a /dev/hdc8  
( this fills in for a missing drive or becomes a spare )

to fail a drive. (useful for testing )
# mdadm /dev/md1 -f /dev/hdc8
mdadm: set /dev/hdc8 faulty in /dev/md1

to remove the failed drive 
# mdadm /dev/md1 -r /dev/hdc8
mdadm: hot removed /dev/hdc8

to start a monitoring process ( sends emails to root if errors occor) 
# mdadm  --monitor /dev/md1 &

to mount the drive: ( you need to have created the filesystem previously)
# mount /dev/md1 /backup

to shut down (stop) a raid array:
# mdadm /dev/md1 -S 
( it must be unmounted first )