I actually hit this for the first time in all my years of using btrfs a few weeks ago.

Every month or so I spin up external drives to write out monthly backups of my NAS and personal computers. Forgot I'd taken on the home video collection and overfilled the array. Deleting some old subvolumes and having the space reclaimed took hours (on a 99.9% full 5 TB RAID-1 volume). Part of that is that I use compress=zstd:10 on the external drives, but I wasn't expecting it. Writes speeds tanked to tens of KB/s. Since these are those Seagate drives from Costco, probably also SMR drives...

Yes! I actually gave up on the one drive. It was a torrent disk (like I said worst case scenario for a lot of things) and btrfs ate itself lol.

To be fair, it didn't break or lose data. Just got really, really slow until I let it finish reclaiming space.

Yeah. On my end it just slowed to a completely unusable level. So while it technically didn’t lose data it effectively did since I was looking at weeks to replicate any data off it. I let it run for a week and it was still crawling when I gave up (4TB drive)

Was it a SMR drive, by a chance? You can make those crawl with any filesystem. I even had one that couldn't finish initial Time Machine backup.

I just double checked the drive and it is. I forgot about that aspect of it.

However I have run this same drive out of space using ext4, zfs, and xfs playing with it (it’s a drive I use for torrent etc) and none of them have crapped out like btrfs did on it.