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

NAME

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

SYNOPSIS

       #include <unistd.h>
       #include <asm/page.h> /* to find PAGE_SIZE */
       #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 only be used by a privileged process (one having the CAP_SYS_ADMIN capability).

   Priority
       Each swap area has a priority, either high or low.  The default priority 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; or, for swapon(), the indicated path does not con-
	      tain a valid swap signature or resides on an in-memory file system like tmpfs; or, for 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.  Alternatively, 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 intro-
       duced 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 conditional on the SWAP_FLAG_DISCARD flag in kernel 2.6.36, which still
       discards 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 3.44 of the Linux man-pages project.  A description of the project, and information about reporting bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux								    2010-11-15								 SWAPON(2)