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 ...