Linux – Configuring a mdadm RAID-1 on SSD and HDD

I found the solution.

I tried fstrim with explicit statements of the partitions as a weekly cronjob shortly after I posted the answer but didn't know whether it was safe. Furthermore, I didn't know whether someone would have a better answer. Therefore, I didn't post it as an answer myself.

I just looked at the contents of the folder /etc/cron.weekly where I placed a file containing the command because I wanted to execute it manually after filling my entire disk with data today and having deleted a lot of it a few minutes ago. I didn't remember the command, so I just went where I put it. However, after executing ll I found 2 files with names leading me to believe that it was my script.

The other file wasn't put there by me so it's almost certainly delivered with Ubuntu. It's called fstrim and these are its contents:

#!/bin/sh
# trim all mounted file systems which support it
/sbin/fstrim --all || true

After deleting my own script, I executed the script which came with Ubuntu because its clearly better. It felt like it took about a minute to execute which is what I'd expect from it trimming 36 GB. When I executed it a second time, it returned immediately, indicating that it did actually trim the first time.

I already sha256summed all the files in my home folder before trimming the first time, sha256summed all of them after the first trim, and found no unexpected changes (well, obviously, the contents of ~/.cache and ~/.mozilla changed as I used Firefox in the meantime) when comparing the files containing the sha256sums via Meld. I therefore concluded that it's probably safe.

The current status of btrfs can be found on the btrfs wiki's “Status” article. btrfs RAID1 has been stable for a while, and has several useful features that mdraid does not (e.g., mirror is per-file, so you can have unmirrored files too if you want—though tools to do this aren't readily available). Also, adding a third drive (when you need more space) is much easier with btrfs than mdraid.

mdraid's main advantage is it's far more mature. So mdraid+ext4 is probably a little bit safer. Documentation & experts are easier to find. But if you're running btrfs on top of it anyway (because, e.g., you want snapshots)….

Personally, I have a set of external drives I use for one layer of backups; they use btrfs. I started with one drive (back in 2014), then later added a second (larger) drive for redundancy. Later the first drive failed, so I replaced it with another larger drive. Then I was going to run out of space, so I added a third drive, twice as big as the other two. Sometime in the next year or so, I'll have to add another. All of this would be much more painful (or at least time-consuming) to do with mdraid than it was with btrfs RAID1.

So, for your application, I'd suggest btrfs RAID1 is a better choice. If you were running a mission-critical darn-near-100% uptime required server, it'd be different. And of course, as always, keep backups (of your media files, at least if you care if they're lost — you don't want to know how many copies of my music collection I have...)