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SWAPON(2)		   Linux Programmer's Manual		     SWAPON(2)

NAME
       swapon, swapoff - start/stop swapping to	file/device

SYNOPSIS
       #include	<unistd.h>
       #include	<sys/swap.h>

       int swapon(const	char *path, int	swapflags);
       int swapoff(const char *path);

DESCRIPTION
       swapon()	 sets  the  swap area to the file or block device specified by
       path.  swapoff()	stops swapping to the file or block  device  specified
       by path.

       If the SWAP_FLAG_PREFER flag is specified in the	swapon() swapflags ar-
       gument, the new swap area will have a  higher  priority	than  default.
       The priority is encoded within swapflags	as:

	   (prio __ SWAP_FLAG_PRIO_SHIFT) _ SWAP_FLAG_PRIO_MASK

       If  the	SWAP_FLAG_DISCARD  flag	is specified in	the swapon() swapflags
       argument, freed swap pages will be discarded before they	are reused, if
       the  swap device	supports the discard or	trim operation.	 (This may im-
       prove performance on some Solid State Devices, but often	it does	 not.)
       See also	NOTES.

       These  functions	 may  be used only by a	privileged process (one	having
       the CAP_SYS_ADMIN capability).

   Priority
       Each swap area has a priority, either high or low.  The default	prior-
       ity  is low.  Within the	low-priority areas, newer areas	are even lower
       priority	than older areas.

       All priorities set with swapflags are high-priority,  higher  than  de-
       fault.	They  may  have	 any  nonnegative  value chosen	by the caller.
       Higher numbers mean higher priority.

       Swap pages are allocated	from areas in priority order, highest priority
       first.	For areas with different priorities, a higher-priority area is
       exhausted before	using a	lower-priority area.  If  two  or  more	 areas
       have the	same priority, and it is the highest priority available, pages
       are allocated on	a round-robin basis between them.

       As of Linux 1.3.6, the kernel usually follows these  rules,  but	 there
       are exceptions.

RETURN VALUE
       On  success,  zero is returned.	On error, -1 is	returned, and errno is
       set appropriately.

ERRORS
       EBUSY  (for swapon()) The specified path	is already  being  used	 as  a
	      swap area.

       EINVAL The  file	 path exists, but refers neither to a regular file nor
	      to a block device;

       EINVAL (swapon()) The indicated path does not contain a valid swap sig-
	      nature or	resides	on an in-memory	filesystem such	as tmpfs.

       EINVAL (since Linux 3.4)
	      (swapon()) An invalid flag value was specified in	flags.

       EINVAL (swapoff()) path is not currently	a swap area.

       ENFILE The  system  limit  on  the  total number	of open	files has been
	      reached.

       ENOENT The file path does not exist.

       ENOMEM The system has insufficient memory to start swapping.

       EPERM  The caller does not have the CAP_SYS_ADMIN capability.  Alterna-
	      tively, the maximum number of swap files are already in use; see
	      NOTES below.

CONFORMING TO
       These functions are Linux-specific and should not be used  in  programs
       intended	 to be portable.  The second swapflags argument	was introduced
       in Linux	1.3.2.

NOTES
       The partition or	path must be prepared with mkswap(8).

       There is	an upper limit on the number of	swap files that	may  be	 used,
       defined	by  the	 kernel	constant MAX_SWAPFILES.	 Before	kernel 2.4.10,
       MAX_SWAPFILES has the value 8; since kernel 2.4.10, it  has  the	 value
       32.  Since kernel 2.6.18, the limit is decreased	by 2 (thus: 30)	if the
       kernel is built with the	CONFIG_MIGRATION option	 (which	 reserves  two
       swap  table entries for the page	migration features of mbind(2) and mi-
       grate_pages(2)).	 Since kernel 2.6.32, the limit	is  further  decreased
       by 1 if the kernel is built with	the CONFIG_MEMORY_FAILURE option.

       Discard of swap pages was introduced in kernel 2.6.29, then made	condi-
       tional on the SWAP_FLAG_DISCARD flag in kernel 2.6.36, which still dis-
       cards  the  entire swap area when swapon() is called, even if that flag
       bit is not set.

SEE ALSO
       mkswap(8), swapoff(8), swapon(8)

COLOPHON
       This page is part of release 3.74 of the	Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at
       http://www.kernel.org/doc/man-pages/.

Linux				  2014-05-28			     SWAPON(2)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON

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