Veritas File System 4.1 Administrator's Guide (HP-UX 11i v3, February 2007)

Application Interface
Cache Advisories
Chapter 4 61
Unbuffered I/O
If the VX_UNBUFFERED advisory is set, I/O behavior is the same as direct I/O with the VX_DIRECT advisory
set, so the alignment constraints that apply to direct I/O also apply to unbuffered I/O. For unbuffered I/O,
however, if the file is being extended, or storage is being allocated to the file, inode changes are not updated
synchronously before the write returns to the user. The VX_UNBUFFERED advisory is maintained on a
per-file-descriptor basis.