Ubuntu Manpage: swapon, swapoff - start/stop swapping to file/device

NAME

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

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #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 to indicate the
       error.

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 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.

STANDARDS

       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 Linux 2.4.10, MAX_SWAPFILES has the value 8;  since
       Linux 2.4.10, it has the value 32.  Since Linux 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 Linux
       2.6.32,  the  limit  is  further  decreased  by  1  if  the  kernel  is  built  with   the
       CONFIG_MEMORY_FAILURE  option.   Since  Linux 5.14, the limit is further decreased by 4 if
       the kernel is built with the CONFIG_DEVICE_PRIVATE option.

       Discard of swap pages was introduced  in  Linux  2.6.29,  then  made  conditional  on  the
       SWAP_FLAG_DISCARD  flag  in  Linux  2.6.36, which still discards the entire swap area when
       swapon() is called, even if that flag bit is not set.