Unbuffered I/O

The I/O behavior of the VX_UNBUFFERED advisory is the same as the VX_DIRECT advisory set with the same alignment constraints as direct I/O. However, for unbuffered I/O, if the file is extended, or storage is allocated to the file, metadata updates on the disk for extending the file are not performed synchronously before the write returns to the user. The VX_UNBUFFERED advisory is maintained on a per-file-descriptor basis.