directio man page on SmartOS

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       directio - provide advice to file system

       #include <sys/types.h>
       #include <sys/fcntl.h>

       int directio(int fildes, int advice);

       The  directio()	function  provides  advice  to	the  system  about the
       expected behavior of the application when accessing  the	 data  in  the
       file  associated	 with the open file descriptor fildes. The system uses
       this information to help optimize accesses  to  the  file's  data.  The
       directio()  function has no effect on the semantics of the other opera‐
       tions on the data, though it may affect the performance of other opera‐

       The  advice  argument  is  kept per file; the last caller of directio()
       sets the advice for all applications using  the	file  associated  with

       Values for advice are defined in <sys/fcntl.h>.

		       Applications  get  the  default	system	behavior  when
		       accessing file data.

		       When an application reads data from a file, the data is
		       first  cached in system memory and then copied into the
		       application's  buffer  (see  read(2)).  If  the	system
		       detects	that  the  application is reading sequentially
		       from a  file,  the  system  will	 asynchronously	 "read
		       ahead"  from the file into system memory so the data is
		       immediately available for the next read(2) operation.

		       When an application writes data into a file,  the  data
		       is  first cached in system memory and is written to the
		       device at a later time (see write(2)).  When  possible,
		       the system increases the performance of write(2) opera‐
		       tions by cacheing the data in memory pages. The data is
		       copied  into  system  memory and the write(2) operation
		       returns immediately to the  application.	 The  data  is
		       later written asynchronously to the device. When possi‐
		       ble, the cached data is "clustered" into	 large	chunks
		       and written to the device in a single write operation.

		       The  system behavior for	 DIRECTIO_OFF can change with‐
		       out notice.

		       The system behaves as though  the  application  is  not
		       going  to  reuse	 the  file data in the near future. In
		       other words, the file data is not cached	 in  the  sys‐
		       tem's memory pages.

		       When possible, data is read or written directly between
		       the application's memory and the device when  the  data
		       is  accessed with read(2) and write(2) operations. When
		       such transfers are not possible,	 the  system  switches
		       back  to the default behavior, but just for that opera‐
		       tion. In general, the transfer  is  possible  when  the
		       application's  buffer  is aligned on a two-byte (short)
		       boundary, the offset into the file is on a device  sec‐
		       tor boundary, and the size of the operation is a multi‐
		       ple of device sectors.

		       This advisory is ignored while the file associated with
		       fildes is mapped (see mmap(2)).

		       The system behavior for	DIRECTIO_ON can change without

       Upon successful completion, directio() returns 0. Otherwise, it returns
       −1 and sets errno to indicate the error.

       The directio() function will fail if:

		 The fildes argument is not a valid open file descriptor.

		 The fildes argument is not associated with a file system that
		 accepts advisory functions.

		 The value in advice is invalid.

       Small sequential I/O generally performs best with  DIRECTIO_OFF.

       Large sequential I/O generally performs best with  DIRECTIO_ON,	except
       when a file is sparse or is being extended and is opened with O_SYNC or
       O_DSYNC (see  open(2)).

       The directio() function is supported for the NFS and  UFS  file	system
       types (see fstyp(1M)).

       See attributes(5) for descriptions of the following attributes:

       │MT-Level       │ MT-Safe	 │

       fstyp(1M),   mmap(2),   open(2),	  read(2),  write(2),  fcntl.h(3HEAD),

       Switching between  DIRECTIO_OFF and  DIRECTIO_ON can  slow  the	system
       because	each  switch  to  DIRECTIO_ON might entail flushing the file's
       data from the system's memory.

				  Apr 9, 2003			  DIRECTIO(3C)

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