Most non-trivial databases run on what is essentially their own purpose-built file system, bypassing many (or all) of the OS file services. Doing so is both hig

Most non-trivial databases run on what is essentially their own purpose-built file system, bypassing many (or all) of the OS file services. Doing so is both higher performance and simpler than just going through the OS file system. Normal OS file systems are messy and complex because they are serving several unrelated and conflicting purposes simultaneously. A database file system has a fairly singular purpose and focused mission, and also doesn't have the massive legacy baggage of general purpose file systems, so there are fewer tradeoffs and edge cases to deal with. The more sophisticated the database kernel, the more the OS is treated like little more than a device driver.