If the VX_DSYNC advisory is set, the user is requesting data synchronous I/O. In synchronous I/O, the data is written, and the inode is written with updated times and, if necessary, an increased file size. In data synchronous I/O, the data is transferred to disk synchronously before the write returns to the user. If the file is not extended by the write, the times are updated in memory, and the call returns to the user. If the file is extended by the operation, the inode is written before the write returns.

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