Direct I/O is an unbuffered form of I/O. If the VX_DIRECT advisory is set, the user is requesting direct data transfer between the disk and the user-supplied buffer for reads and writes. This bypasses the kernel buffering of data, and reduces the CPU overhead associated with I/O by eliminating the data copy between the kernel buffer and the user's buffer. This also avoids taking up space in the buffer cache that might be better used for something else. The direct I/O feature can provide significant performance gains for some applications.

The direct I/O and VX_DIRECT advisories are maintained on a per-file-descriptor basis.