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
       argument, 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
       improve 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
       default.	  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(5).

       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-wide 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
       migrate_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 4.10 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
       https://www.kernel.org/doc/man-pages/.



Linux				  2016-12-12			     SWAPON(2)