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
   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;

   EINVAL (swapon()) The indicated path does  not  contain  a  valid  swap
          signature   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.
          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 4.09 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/.

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