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.
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 4.10 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/.