I just tried to use
btrfs send for a backup but it ended up giving me an io error. I ran
btrfs scrub, which reported 32 unrepairable blocks on my raspi's sd card.
The obvious way to fix this is to get fresh copies of the affected files, but how do you find out which files they are?
Everywhere I've looked online, wikis and people only ever mention that you can do a scrub and that typically everything ends well if you have raid, not how to get to the next step after being told there are errors.