The directio parameter for disk init and disk reinit, lets you bypass the operating system buffer cache and transfer Adaptive Server data directly to disk.
directio performs I/O in the same manner as raw devices and provides the same performance benefit, but has the ease of use and manageability of file system devices.
By default, the directio option is set to true (on) for all platforms. directio and dsync are ignored for raw devices.
See the System Administration Guide, Volume 1.