Dynamically Convert A Raid5 Array to Raid6

Transferred from my old blog:
If you have the newest mdadm tool, version 3.1.1 and above, it is now capable of changing the raid level of an array. If you cannot find a copy of the latest version for your distro you can compile from source via mdadm’s git repo, git://neil.brown.name/mdadm
The below assumes that you have 1 spare drive ready to add to your array (/dev/sdb) and that /dev/md0 is the raid5 array that you would like to move to raid6 and that you have 4 raid devices in /dev/md0 to start with.
Use the below commands:

mdadm --add /dev/md0 /dev/sdb
mdadm --grow --level=6 --raid-devices=5

Once this completes, you should have a fully functioning raid6 array. Enjoy your dual parity.
Further, you can also change the chunk size dynamically while you’re at it, the default chunk size of mdadm (which I believe they plan to up in future versions) is a paltry 64k, you’d be much better off with something in the 256-512k range. To change the chunk size of an array, use the following:

mdadm --grow /dev/md0 --chunk=512

I’ve seen several references now using –chunks-size, so it’s possible in future versions this may be the correct flag instead of –chunk, just something to be aware of. Also, upping your chunk size to 512 may not be possible depending what the total size of your array is. It’s possible that mdadm will spit out an error stating that the total array size is not divisible by 512, in which case you’ll have to settle for something smaller. (i.e. try 256 or 128).