Auto-repair on read
Data or metadata that are found to be damaged (e.g. because the checksum does not match) at the time they’re read from a device can be salvaged in case the filesystem has another valid copy when using block group profile with redundancy (DUP, RAID1-like, RAID5/6). The correct data are returned to the user application and the damaged copy is replaced by it. When this happen a message is emitted to the system log.
If there are more copies of data and one of them is damaged but not read by user application then this is not detected. To verify all data and metadata copies there’s scrub that needs to be started manually, automatic repairs happens in that case.