swapon, swapoff - start/stop swapping to file/device
int swapon(const char *path, int swapflags);
int swapoff(const char *path);
() sets the swap area to the file or block device specified by
() stops swapping to the file or block device
specified by path
If the SWAP_FLAG_PREFER
flag is specified in the swapon
argument, the new swap area will have a higher priority than
default. The priority is encoded within swapflags
(prio << SWAP_FLAG_PRIO_SHIFT) & SWAP_FLAG_PRIO_MASK
If the SWAP_FLAG_DISCARD
flag is specified in the swapon
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
These functions may be used only by a privileged process (one having the
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
All priorities set with swapflags
are high-priority, higher than default.
They may have any nonnegative value chosen by the caller. Higher numbers mean
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
On success, zero is returned. On error, -1 is returned, and errno
- (for swapon()) The specified path is already being used as a
- The file path exists, but refers neither to a regular file nor to a
- (swapon()) The indicated path does not contain a valid swap
signature or resides on an in-memory filesystem such as
- EINVAL (since Linux 3.4)
- (swapon()) An invalid flag value was specified in
- (swapoff()) path is not currently a swap area.
- The system-wide limit on the total number of open files has been
- The file path does not exist.
- The system has insufficient memory to start swapping.
- The caller does not have the CAP_SYS_ADMIN capability.
Alternatively, the maximum number of swap files are already in use; see
These functions are Linux-specific and should not be used in programs intended
to be portable. The second swapflags
argument was introduced in Linux
The partition or path must be prepared with mkswap
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,
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)). Since kernel 2.6.32, the limit is further decreased
by 1 if the kernel is built with the CONFIG_MEMORY_FAILURE
Discard of swap pages was introduced in kernel 2.6.29, then made conditional on
flag in kernel 2.6.36, which still discards the
entire swap area when swapon
() is called, even if that flag bit is not