(Since Linux 2.4.10) Try to minimize cache effects of the I/O to and from this file. In general this will degrade performance, but it is useful in special situations, such as when applications do their own caching. File I/O is done directly to/from user space buffers. The +o-direct+ flag on its own makes an effort to transfer data synchronously, but does not give the guarantees of the +o-sync+ flag that data and necessary metadata are transferred. To guarantee synchronous I/O, +o-sync+ must be used in addition to +o-direct+. See NOTES in open for further discussion.

A semantically similar (but deprecated) interface for block devices is described in raw(8).