Distributed in-memory storage systems are crucial for meeting the low latency requirements of modern datacenter services. However, they lose all state on failure, so recovery is expensive and data loss is always a risk. The Persimmon system leverages persistent memory (PM) to convert existing in-memory storage systems into persistent, crash-consistent versions with low overhead and minimal code changes. Persimmon offers a simple Persistent State Machine abstraction for PM and implements persistence through operation logging on the critical path and a novel crash-consistent shadow execution technique for log digestion in the background.