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 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 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 contain a valid swap signature or resides on an in-memory
              filesystem 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 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 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 3.54 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/.