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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  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.

     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 sig-
            nature 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.  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.