I come across this phrase from Kafka Definitve Guide
Produced messages are considered “committed” when they were written to the partition on all its in-sync replicas (but not necessarily flushed to disk).
Understading more about the phrase but not necessarily flushed to disk , How to interpret this ? What should be interpreted in case if message is lost during the time even before it is flushed to the disk or is it that message is guaranteed to be persisted to the disk from in memory during segment processing.