It just needs to listen to explicit fsync commands and choose a matching point-in-time. Beyond that, no, it's not hard as a database to tell the OS about moment

It just needs to listen to explicit fsync commands and choose a matching point-in-time. Beyond that, no, it's not hard as a database to tell the OS about moments where a block-level disk image would not need repairing, after the OS asks you. I assume this is to prevent situations that would need time-intensive WAL-replay or such, and there it'd only be opportunistic with the "sudden power loss" recovery as a fall-back path.