Direct I/O Writes: The Path to Storage Wealth

As storage technology evolves with the advent of fast NVMe devices, the traditional preference for Buffered I/O, where the operating system caches data pages, is being challenged, particularly when contrasted with Direct I/O, which bypasses such caching. Glauber Costa, in his article, argues that Direct I/O offers more reliable and predictable performance by writing data directly to the device, thereby avoiding the pitfalls of delayed costs associated with Buffered I/O, which can lead to increased CPU usage and memory pressure. Using examples from the Glommio io_uring asynchronous executor for Rust, Costa illustrates that Direct I/O not only reduces the illusion of cheaper access created by Buffered I/O but also ensures real-time data persistence without the unpredictability of kernel thread operations. While Buffered I/O might initially appear faster due to its ability to temporarily leverage system memory, it ultimately incurs higher costs and risks as file sizes increase, making Direct I/O the preferred choice for achieving efficient and sustainable storage performance.