swapon


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  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  prior-
       ity  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.

       ENOMEM The system has insufficient memory to start swapping.

       EPERM  The caller does not have the CAP_SYS_ADMIN capability.  Alterna-
              tively, 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 condi-
       tional on the SWAP_FLAG_DISCARD flag in kernel 2.6.36, which still dis-
       cards  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 3.54 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.



Linux                             2010-11-15                         SWAPON(2)
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