Okay, so here's the recap:

I woke up this morning and decided my main drive (just a 500GB SSD) was too full, at about 85%, so I decided to do something about that. I go through the usual: pacman -Sc, paccache -rk0, and pacman -Qqtd | pacman -Rns - (which I've aliased to "orphankiller" because that's too much typing for me). None of that did anything though, as I'm usually pretty up on this, and I expected it, so my next step was to find other ways of deleting unnecessary files floating around, and that meant a trip to the usually very helpful Arch wiki.

On the page "pacman Tips and Tricks", I find 1.7: Detecting More Unneeded Packages. "Perfect!" I thought, "That's exactly what I'm looking for!" I enthusiastically type in the command pacman -Qqd | pacman -Rns -, and then quickly go check how much space I just saved. Nada. Or at least not enough to move the percentage point. "Oh well, keep looking," I think and I go back to Firefox to click some more links in hopes that one of them will be the space saving ultra-script that I need. The first one I click, I get an error from my trusty browser, I don't remember exactly what it was but it was something about not being able to verify the page. "Weird, let's try another one." Nope, same thing.

Well, being that I had just deleted something, I figured I should go see what exactly it was that I did. It was a good thing I'd left the terminal window open, because after just a few scrolls I saw it: ca_certificates, which Firefox absolutely needs. "Great, I'll just reinstall." Nope! I just deleted my pacman cache, and pacman also needs those certificates to download from the Arch repo's mirrors! "Fantastic," I grumbled while I tried to think of how I could get this pesky package back on my machine.

Then it occurred to me: I've been keeping up with my btrfs snapshots (for once, lol)! I can just backup to yesterday and forget this whole mess! So I bring up Timeshift, and we're on our way back to a functioning system! Or so I thought. See, I don't have a separate /home partition, but I do have a separate @home subvolume, so when Timeshift asked me if I wanted to restore that too, I clicked the check mark. Only thing is, I don't think I actually have a separate @home subvolume, which brings us to the error in the meme. /home wouldn't mount, and that meant I was borked.

Fortunately, our story has a happy ending! I DDG'd the error on my phone, and found a post from like seven years ago, about someone who had this same set of circumstances, and the one reply was my fix: just go into /etc/fstab and delete the "subvolid" part of whatever partition that's giving you grief. Did that, reboot, and we're finally fixed! And now, forevermore, I shall check what I'm deleting before I hit the enter button!

The post-script is bittersweet though, because after all this trouble, and then the rest of the afternoon working on the original problem, I am down to... 81%. Oh well.

Please do share. What better thing to do than to take a break from a broken install to read about someone's own hardship with the endless quest that is maintaining a rolling-release distro.

this happened to me once, oh man.... it was painful

Just an update: following the very helpful suggestions in this thread has gotten my drive usage down to 16%! Super happy about that, y'all rock!

Sometimes, when it backstabs me, I call it BeTRayFS

I use qdirstat a lot to determine what files are eating all my space

Definitely want to hear the story! I’ve been through my own version of this and would like to hear your experience and how you resolved it.

I don't understand why people use Arch. It takes all kinds I suppose. For me I automate everything and use preconfigured stuff when I can.

sound's like timeshift's fault, not btrfs

I've been burned by btrfs before. Never again. It's not a good file system, especially for multi disk systems.

My moment was using the experimental repos to get an early view into wayland, after seeing it wasn't quite ready for my system I just switched back. Mistakes made, and slowly over the next few weeks as I updated, the experimental packages never got superseded and updated, until my system crashed and would not pass boot.

Luckily since it is not windows I just used a live usb stick to mount the disk and manually reinstall all the broken system packages. Scary but made me feel pretty confident I could recover the system myself in the future. Also learned a pretty important lesson. Don't do that, and look at the upgrade log if you do lol, cause the whole time, as I upgraded there was red text showing me all the system packages that were not getting updated.

This is an interesting read, even if it is a few years old https://arstechnica.com/gadgets/2021/09/examining-btrfs-linuxs-perpetually-half-finished-filesystem/

I gave up on it in in 2016 and it sounded all the same back then too with too many people giving it a pass for unacceptable behavior. I don't think anything has really changed since.