SWAPON
Section: Linux Programmer's Manual (2)
Updated: 2017-09-15
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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.
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 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 4.13 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/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- Priority
-
- RETURN VALUE
-
- ERRORS
-
- CONFORMING TO
-
- NOTES
-
- SEE ALSO
-
- COLOPHON
-
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