Function degraded_syncs 

pub fn degraded_syncs() -> u64

How many syncs have fallen back to fsync(2).

Zero on a volume that implements write barriers, which is every volume Mira is meant to run on. Non-zero means the durability promise on this node is weaker than the one sync_all documents, and it is reported by /api/v1/stats so that is a fact an operator can see rather than infer.