Isn't that the entire point of write-ahead logs, journaling file systems, and fsync in general? A roll-back or roll-forward due to a power loss causing a partial write is completely expected, but surely consumer SSDs wouldn't just completely ignore fsync and blatantly lie that the data has been persisted?

As I understood it, the capacitors on datacenter-grade drives are to give it more flexibility, as it allows the drive to issue a successful write response for cached data: the capacitor guarantees that even with a power loss the write will still finish, so for all intents and purposes it has been persisted, so an fsync can return without having to wait on the actual flash itself, which greatly increases performance. Have I just completely misunderstood this?