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

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

       #include <unistd.h>
       #include <sys/swap.h>

       int swapon(const char *path, int swapflags);
       int swapoff(const char *path);

       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

       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:


       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).

       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

       On success, zero is returned.  On error, -1 is returned, and errno is set
       to indicate the error.

       EBUSY  (for swapon()) The specified path is already being used as a swap

       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 swapflags.

       EINVAL (swapoff()) path is not currently a swap area.

       ENFILE The system-wide limit on the total number of open files has been

       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.

       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.

       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.

       mkswap(8), swapoff(8), swapon(8)

       This page is part of release 5.11 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

Linux                              2021-03-22                          SWAPON(2)