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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
              contain  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 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 3.35 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://man7.org/linux/man-pages/.