Editing Linux RAID Array

== Creating a new RAID array ==
If it isn't already installed, load the mdadm package:
 yum -y install mdadm

Get the list of disks in the system and identify the ones that you want to include:
 fdisk -l | grep GB

Identify if any arrays are already in place:
 cat /proc/mdstat

Create the array -- generally, <mdevice> will start with /dev/md0
 mdadm --create <mdevice> --chunk=64K --level=5 --raid-devices=<count> <list of drive devices>

If you get I/O errors, try chunk sizes in powers of 2 starting at 4K -- larger is better, but exactly divides the available space (i.e. you may waste space on each device that is less than a chunk).  The errors may not come when creating the array; creating the filesystem is where the errors could occur.

 mdadm --create <mdevice> --chunk=<chunksize> --level=5 --raid-devices=<count> <list of drive devices>

Check the status of the array:
 cat /proc/mdstat

If you'll be mounting it directly, then create a filesystem on it:
 mkfs -t xfs <mdevice>

== Replacing a failing disk ==
Identify the disks involved:
* /dev/sdY - new disk (or partition /dev/sdY1)
* /dev/sdX - old failing disk

If replacing with a partition of a disk
 mdadm /dev/mdN --add /dev/sdY1 --replace /dev/sdX --with /dev/sdY1

If replacing with a whole disk
 mdadm /dev/mdN --add /dev/sdY1 --replace /dev/sdX --with /dev/sdY


== Removing an MDADM Raid Array ==

Find out your arrays (md0, md1, etc..) using
 sudo fdisk -l

Query your arrays to find out what disks are contained using
 sudo mdadm --detail /dev/md0

... using the appropriate /dev/md* device ...

Unmount the array first, otherwise it fails with "device is busy".
 sudo umount /dev/md0

Shut down the array using
 sudo mdadm --stop /dev/md0

And here's the magic key:  zero the superblock FOR EACH drive so it doesn't try to rebuild the array
 sudo mdadm --zero-superblock /dev/sdX

Repeat for each drive in the array ...