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In addition to that, SATA/SAS drives also have a cache disable command. When you disable the cache Linux stops sending flushes at all. It may seem that this should also result in the same performance as fsync/O_SYNC, but that’s not the case either! SSDs with supercaps give '''much''' better performance with disabled cache. For example, Seagate Nytro 1351 gives you 288 iops with cache and 18000 iops without it (!).
Why? It seems that’s because FLUSH CACHE is interpreted by the drive as a «please flush all caches, including non-volatile cache» command, and «disable cache» is interpreted as «please disable the volatile cache, but you may leave the non-volatile one on if you want to». This makes writes with a flush after every write slower than writes with the cache disabled.
What about NVMe? NVMe has slightly less variability — there is no «disable cache» command in the NVMe spec at all, but just as in the SATA spec there is the FLUSH CACHE command and FUA bit. But again, based on the personal experience I can say that it seems that FUA is often ignored with NVMe either by Linux or by the drive itself, thus '''fio -sync=1''' gives the same results as '''fio -direct=1''' without any sync flags. '''-fsync=1''' performs correctly and lands the performance down to where it must belong (1000—2000 iops for desktop NVMes).
