Convert into a RAID5 array:
mdadm –grow /dev/md3 –level 5

Add a hot spare(s):
mdadm –manage /dev/md3 –add /dev/sdc3

Add a disk to the array, to get a clean state for the array:
mdadm –grow /dev/md3 –raid-devices 3

Online resize to fill partition
resize2fs /dev/md3

I think I got some kind of weird warning/error at some point there, but it worked fine, and was able to be mounted during rebuild / reshape. Then, as I got more adventurous, I tried this with my 400GB partition. I got some message like, “Unable to set to raid 5″. (That’s not verbatim… but something like that.) I looked it up on Google and had very few results, none of which helped me. I tried unmounting it and doing the same thing, but no. It just wouldn’t do anything with my RAID partition, and /var/log/messages really didn’t help either. Then, I stumbled upon this site, which gives a slightly older approach, that requires unmounting the volume and stopping it, prior to newly creating the RAID 5 one. Ok, fine, I want/need to do this, so I went into init 3 and tried it out. The part where you run this (in my case):

mdadm –create /dev/md5 –level=5 -n 2 /dev/sda5 /dev/sdb5

… started rebuilding the array in RAID5, but I got e2fsck errors when I tried to run it, and was unable to mount it, whereas this blog says it mounted just fine. It said something about how the superblocks claimed the filesystem (?) was slightly larger than the partition, er something. I thought, “OK, well it will probably fix itself once it’s done rebuilding. Then, shortly after I thought that, I suspected otherwise, and my latter suspicions were right. e2fsck didn’t like it when I ran it with -n during rebuild or normally, but after the rebuild I ran resize2fs and it resized to fill the partition which was just a couple more blocks. (I can only assume the extra space has something to do with RAID 5. The resize that doubles the space is still yet to happen….) Then, it mounts just fine, all RAID1 data intact. So, I boot back into graphical mode and proceed with the rest of the process outlined here, with the “–grow” and stuff. Now I’m golden. I run another resize2fs to make it fill the partition, and it starts showing more and more space available as I run “df” to check it. Point being, you may run into an issue where it’s just not going to mount properly until you resize it, even during the initial setup or RAID 5.

So, I now have my RAID1 —> RAID5 transition that worked fine, but needed some slight adjustements from this (admittedly old) process described here. Not sure if it’s because of a newer mdadm or what, but yes, it can be done. So, good luck.

Infinality

Setting up the 2x500Gb originally as RAID1 -> RAID5 was easy… adding the 3rd 500Gb drive later took a LOT of time for the array to rebuild (was a slow Dual PIII/1.2Ghz machine mind). but otherwise no problems.