> fsync() is required to ensure that the data is on disk, though I've seen systems lie about that too. Virtually all SSDs lie about this to a certain extent. T

> fsync() is required to ensure that the data is on disk, though I've seen systems lie about that too. Virtually all SSDs lie about this to a certain extent. They'll indicate a write is complete when it's in their cache. The cache is RAM; as long as the power stays on, the write will appear to have succeeded to readers, but if the power goes goes off before the drive flushes the cache, the write will disappear. The exception is high end enterprise drives, which come with a capacitor that stores enough power to flush the cache to persistent storage in the event of power loss.