So I built a server for all my media streaming and storage needs. It's got three 3TB WD Reds in it configured in a RAID 5. I'm running Ubuntu 14.04.4 LTS. I had a friend set up the raid for me (I'm not too Linux savvy) about a year ago. Well, I'm running low on space. So I bought 3 more WD reds, same size to add to this machine. The motherboard supports up to 6 drives so that should be good (Ubuntu is running off a thumb drive). I have backed up everything critical (pictures and home movies). If I lose the rest of the data I'll be sad and have lost a LOT of work but it won't be the end of the world. Still, if there's a way to add these new drives to the array without losing everything that'd be great. Can anyone point me to an idiot's guide? Thanks! Any questions about the setup just let me know!
Straight from the mdadm man page.
Grow (or shrink) an array, or otherwise reshape it in some way. Currently supported growth options including changing the active size of component devices and changing the number of active devices in Linear and RAID levels 0/1/4/5/6, changing the RAID level between 0, 1, 5, and 6, and between 0 and 10, changing the chunk size and layout for RAID 0,4,5,6, as well as adding or removing a write-intent bitmap.
Back to your config. Three drives, so two data + one parity (distributed). The more drives you add, the higher the probability that more than one can fail at a time.
If it were me, I would start over with a RAID 6 and get an additional disk to function as a hot spare. Even if that meant purchasing a storage controller. RAID 6 uses two disks worth of distributed parity so your available space would be 4 drives worth.
A compromise would be RAID 5 with a hot spare, which would fit in your current storage controller footprint. The exposure here is that you can take another fault while the array is rebuilding with the spare at which point you've lost everything.
All this is great, but even RAID with spares doesn't mean much without monitoring. We just had a HW RAID go at work, RAID 6 with no spares and no alerts setup. It lost both parity drives and then hit a single unrecoverable read error and that was it, the whole array was lost.
When you take on a software RAID you also take on all the responsibility of running it smoothly. If that isn't for you then I would suggest looking at Drobo storage arrays.
I hope this helps. Losing data sucks... Good luck!