swapon(2) — Linux manual page


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

NAME         top

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

SYNOPSIS         top

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

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

DESCRIPTION         top

       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:


       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 pri‐
       ority 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 de‐
       fault.  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 prior‐
       ity 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 avail‐
       able, 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         top

       On success, zero is returned.  On error, -1 is returned, and errno is
       set appropriately.

ERRORS         top

       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

       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 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         top

       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         top

       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

       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         top

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

COLOPHON         top

       This page is part of release 5.09 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

Linux                            2017-09-15                        SWAPON(2)

Pages that refer to this page: syscalls(2)systemd.exec(5)capabilities(7)swapoff(8)swapon(8)