Direct I/O is a file access mode that transfers data between an application and storage with minimal reliance on the kernel page cache. Instead of caching reads and writes in memory first, the filesystem tries to move data more directly to disk or from disk, which can reduce copying and improve predictability for some workloads.
In cyber security, Direct I/O matters because it pushes more responsibility onto filesystem metadata and block mapping logic. If the filesystem mishandles block ownership, synchronization, or copy-on-write decisions, a write can hit the wrong location or act on stale state. Attackers may look for those edge cases to corrupt privileged files or bypass intended access boundaries. Defenders care because fixing the kernel, validating filesystem layouts, and isolating sensitive files on separate volumes can reduce exposure. In short, Direct I/O is fast, but it depends on the filesystem being correct under concurrent writes and changing mappings.