Scrub checks every data and metadata block and compare to checksum. If missing, or wrong transid, or wrong csum, btrfs will replace the block from a good copy.

Scrub checks every data and metadata block and compare to checksum. If missing, or wrong transid, or wrong csum, btrfs will replace the block from a good copy. There is a potential gotcha with parity being wrong following a crash or powerfail, the so-called write hole. Scrub doesn't check parity, and parity isn't checksummed. The write hole though is arguably two parts: wrong parity and propagated wrong reconstruction. In the usual write-hole case, both happen silently. Wrong parity could happen after a power fail, crash, misdirected or torn write - whether the array is functioning normally or degraded. However, wrong reconstruction only happens if there's a failure to read a data strip (bad sector or full device failure). On btrfs, only the wrong parity being written is possible. Upon reconstruction from bad parity, the resulting data is compared to csum and will fail, thus propagation doesn't happen. To cause parity to be recomputed and rewritten, yes you need to do a full balance. But among all the block group profiles, that's only raid5 and raid6. Single, DUP, raid1, raid1c3, raid1c4, raid10 aren't affected and a scrub does reallocate missing blocks on mirrors.