I recently acquired two used blade servers and a short rack to put them in. I'm planning to use one or the other as the replacement for a media server that died on me a bit ago. The old media server was just a little refurb dell workstation, with a single SSD in it, but the servers have 6 and 8 bays, respectively.
I would like to RAID them so that one drive dying doesn't lose any of my media, and I was leaning towards Ubuntu server as an OS. I'm not sure how to do that, and I'm kind of poking around for info and advice. Hit me with it.
It's sounds like what you're looking for is backup, and RAID is not backup, it's redundancy to maintain uptime (as well as data integrity, and in some cases performance). I'd highly recommend you look into backup options, with best case being a 3-2-1 backup strategy.
To be fair, I'm being a little hypocritical. I've been working on my backup strategy for years and still don’t have any remote backups yet. Personally, I have a JBOD system, with 8 drives ranging from 2TB to 8TB, so my setup might be a bit complicated for your purposes. I’m not worried about uptime, and am focused on data integrity. I’m not using actual RAID because in the case of a catastrophic failure, I don’t want to lose all my data. I use snapRAID to create some redundancy, and I pool my data drives using mergerfs.
If you are still interested in RAID, I would recommend staying away from hardware RAID as I’ve commented in other places of this post. It has it’s place in data centres but really doesn’t make sense for consumers anymore. There is a lot of good advice in the rest of the comments about RAID, so I’ll summarize my thoughts. If you only plan on having 2 drives, RAID 1 is a good option, though it’s generally used for it’s write performance and that’s probably not necessary on a media server. My current server is running on decade old, lower-end, consumer hardware, and even in that extreme case, media sometimes takes a second to start 1080p content remotely. If you want to add drives and are willing to expand in redundant pairs, you you can either add another RAID layer (RAID 1+0), or pool the partitions together. If you want to be able to expand by single drives/have more than 50% of your potential storage be realized, you could look at RAID 5/6 or ZFS/btrfs. Note that for RAID 5/6, drives need to be equal size.
Since I'll have 8 drives (or 6, if I use the smaller server, it would be nice if I could swap out one of them without losing data and add a larger one, which would then get used automatically. Is that something that RAID is good for?
I'm hesitant to set up backups because it's going to be a lot of data.
Sure, that's what RAID is designed to do. However, I'd suggest also looking into what happens when your array is degraded and how to rebuild it.
Fast reads
I'm a bit surprised you need fast reads with a media server. You're probably going to have to clarify your needs a bit more.
Easy plug and play expansion
Since I’ll have 8 drives (or 6, if I use the smaller server, it would be nice if I could swap out one of them without losing data and add a larger one, which would then get used automatically. Is that something that RAID is good for?
Standard RAID levels generally don't have options to add larger drives. I'm not sure what you mean by "plug and play". I'm pretty sure almost all setups will involve a fair bit of configuration.
I’m hesitant to set up backups because it’s going to be a lot of data.
It's also a lot of data to lose if things go more wrong than you expected (multi-drive failure, bit-rot, etc.).