O_DIRECT is separate from synchronization. There is no guarantee that O_DIRECT writes are durable, though a subset of hardware may work this way in fact. The p

O_DIRECT is separate from synchronization. There is no guarantee that O_DIRECT writes are durable, though a subset of hardware may work this way in fact. The practical purpose of O_DIRECT is to have precise visibility and control over what is in memory, what is on disk, and any inflight I/O operations. This opens up an entire category of workload-aware execution scheduling optimizations that become crucial for performance as storage sizes increase.