1SWAPON(2) Linux Programmer's Manual SWAPON(2)
2
6 swapon, swapoff - start/stop swapping to file/device
7
9 #include <unistd.h>
10 #include <asm/page.h> /* to find PAGE_SIZE */
11 #include <sys/swap.h>
12 int swapon(const char *path, int swapflags);
14 int swapoff(const char *path);
15
17 swapon() sets the swap area to the file or block device specified by
18 path. swapoff() stops swapping to the file or block device specified
19 by path.
20 If the SWAP_FLAG_PREFER flag is specified in the swapon() swapflags
22 argument, the new swap area will have a higher priority than default.
23 The priority is encoded within swapflags as:
24 (prio << SWAP_FLAG_PRIO_SHIFT) & SWAP_FLAG_PRIO_MASK
26 If the SWAP_FLAG_DISCARD flag is specified in the swapon() swapflags
28 argument, freed swap pages will be discarded before they are reused, if
29 the swap device supports the discard or trim operation. (This may
30 improve performance on some Solid State Devices, but often it does
31 not.) See also NOTES.
32 These functions may be used only by a privileged process (one having
34 the CAP_SYS_ADMIN capability).
35 Priority
37 Each swap area has a priority, either high or low. The default prior‐
38 ity is low. Within the low-priority areas, newer areas are even lower
39 priority than older areas.
40 All priorities set with swapflags are high-priority, higher than
42 default. They may have any nonnegative value chosen by the caller.
43 Higher numbers mean higher priority.
44 Swap pages are allocated from areas in priority order, highest priority
46 first. For areas with different priorities, a higher-priority area is
47 exhausted before using a lower-priority area. If two or more areas
48 have the same priority, and it is the highest priority available, pages
49 are allocated on a round-robin basis between them.
50 As of Linux 1.3.6, the kernel usually follows these rules, but there
52 are exceptions.
53
55 On success, zero is returned. On error, -1 is returned, and errno is
56 set appropriately.
57
59 EBUSY (for swapon()) The specified path is already being used as a
60 swap area.
61 EINVAL The file path exists, but refers neither to a regular file nor
63 to a block device; or, for swapon(), the indicated path does not
64 contain a valid swap signature or resides on an in-memory file
65 system like tmpfs; or, for swapoff(), path is not currently a
66 swap area.
67 ENFILE The system limit on the total number of open files has been
69 reached.
70 ENOENT The file path does not exist.
72 ENOMEM The system has insufficient memory to start swapping.
74 EPERM The caller does not have the CAP_SYS_ADMIN capability. Alterna‐
76 tively, the maximum number of swap files are already in use; see
77 NOTES below.
78
80 These functions are Linux-specific and should not be used in programs
81 intended to be portable. The second swapflags argument was introduced
82 in Linux 1.3.2.
83
85 The partition or path must be prepared with mkswap(8).
86 There is an upper limit on the number of swap files that may be used,
88 defined by the kernel constant MAX_SWAPFILES. Before kernel 2.4.10,
89 MAX_SWAPFILES has the value 8; since kernel 2.4.10, it has the value
90 32. Since kernel 2.6.18, the limit is decreased by 2 (thus: 30) if the
91 kernel is built with the CONFIG_MIGRATION option (which reserves two
92 swap table entries for the page migration features of mbind(2) and
93 migrate_pages(2)). Since kernel 2.6.32, the limit is further decreased
94 by 1 if the kernel is built with the CONFIG_MEMORY_FAILURE option.
95 Discard of swap pages was introduced in kernel 2.6.29, then made condi‐
97 tional on the SWAP_FLAG_DISCARD flag in kernel 2.6.36, which still dis‐
98 cards the entire swap area when swapon() is called, even if that flag
99 bit is not set.
100
102 mkswap(8), swapoff(8), swapon(8)
103
105 This page is part of release 3.53 of the Linux man-pages project. A
106 description of the project, and information about reporting bugs, can
107 be found at http://www.kernel.org/doc/man-pages/.
108Linux 2010-11-15 SWAPON(2)