btrfs -- what happens when a checksum fails on a single disk?

btrfs -- if you have your data stored multiply (ie. raid mirror) then if one copy goes bad, it'll resync it from the other copy. That's all well and good. Can you explain the failure modes for when a block checksum fails when you have data on a single disk? (I gather that there are both block and extent checksums, but correct me if I'm wrong) Essentially I was wondering whether you'd lose an entire file (or extent of a file) if the checksum fails, or whether you could still access the rest of the file but with just one block missing in the event of a bad sector. Cheers, Toby