Synchronous block level replication across availability zones

A replicated block storage service provides durable and high performance network-attached storage replicated in two or more zones of a single region, and remains available despite a single zone failure. A probe file is generated to determine a health state of a replicated disk. When a disk is degraded, a lease is created indicating which replica is trusted and providing visibility to backend jobs to facilitate reconciliation of data between the first replica and the second replica. Moreover, degraded file markers are generated for use by the backend jobs in quickly identifying the data to be copied.