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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON |
SWAPON(2) Linux Programmer's Manual SWAPON(2)
swapon, swapoff - start/stop swapping to file/device
#include <unistd.h>
#include <sys/swap.h>
int swapon(const char *path, int swapflags);
int swapoff(const char *path);
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.
On success, zero is returned. On error, -1 is returned, and errno is
set appropriately.
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.
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.
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.
mkswap(8), swapoff(8), swapon(8)
This page is part of release 4.12 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 2016-12-12 SWAPON(2)
Pages that refer to this page: syscalls(2), systemd.exec(5), capabilities(7), swapon(8)