Finally finished migration. The difference is clear: I’m barely getting 20 writes per second, when I used to have even 200 on btrfs, when comparing a similar period in terms of a number and size of upload requests. I didn’t expect the difference will be so huge.
One thing is that the btrfs partition was accidentally created with metadata in DUP scheme, which probably explains half of the difference. Maybe being log-structured might explain the other half?
So while I don’t have hard evidence against btrfs, I’d probably side towards advising against it for storage nodes.