swapoff

SWAPON(2)                  Linux Programmer's Manual                 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 5.04 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/.




Linux                             2017-09-15                         SWAPON(2)