Software RAID

Installation

The mdadm package is required for software RAID:

The mdadm package is available on most distributions (mdadm on Fedora/Arch/Debian).

Array Creation

mdadm --create /dev/md0 --verbose --level=1 --raid-devices=2 /dev/sda /dev/sdb

Shorthand:

mdadm -Cv /dev/md0 -l1 -n2 /dev/sd[ab]

Note: If you use a RAID 0 and want to put ZFS on top of be sure to set a chunk size <= 256 (maybe even 128 if you're still getting errors) otherwise ZFS will warn about issues creating it's partitions.

Troubleshooting

Status Check

You can view the status of the RAID by cat'ing /proc/mdstat. You can see more details by using the mdadm utility like so:

mdadm --misc --detail /dev/md0

As long as the state is clean you're golden.

Recovery

Configuration File

During initial setup the /etc/mdadm.conf is created automatically. All this data exists in the metadata on the disks and can be rebuilt with the mdadm tool like so:

mdadm --examine --scan > /etc/mdadm.conf

Remove Disk from Array

A disk needs to be failed before it can be removed from an array, if it isn't already you'll need to fail it manually:

mdadm /dev/md0 --fail /dev/sda

Then remove it:

mdadm /dev/md0 --remove /dev/sda

Or in a single step:

mdadm /dev/md0 --fail /dev/sda --remove /dev/sda

Adding a Disk to an Existing Array

Probably useful for replacing a failed disk:

mdadm /dev/md0 --add /dev/sda

Delete an Array

You'll lose all data... don't say I didn't warn you...

mdadm --stop /dev/md0

I didn't need the second command but you'll want to run it if the device is still kicking around:

mdadm --remove /dev/md0

And blow away the super block on all of the drives:

mdadm --zero-superblock /dev/sd[ab]