Fsync

A narrow, single-node KV engine removes fsync from the durable path using fixed-size preallocated files, O_DIRECT writes, and a 4KB-aligned journal with atomic commits, delivering roughly 2x higher 4KB-write throughput and lower tail latency on NVMe, but requiring a narrow SSD durability contract and not intended as a general-purpose KV or transaction engine.

Jepsen finds NATS JetStream 2.12.1 can lose committed writes and exhibit persistent split-brain under faults, largely due to lazy fsync and on-disk file corruption, with prior related issues fixed in 2.10.23.