swapon(2)

1swapon(2)                     System Calls Manual                    swapon(2)
2
3
4

NAME

6       swapon, swapoff - start/stop swapping to file/device
7

LIBRARY

9       Standard C library (libc, -lc)
10

SYNOPSIS

12       #include <sys/swap.h>
13
14       int swapon(const char *path, int swapflags);
15       int swapoff(const char *path);
16

DESCRIPTION

18       swapon()  sets  the  swap area to the file or block device specified by
19       path.  swapoff() stops swapping to the file or block  device  specified
20       by path.
21
22       If the SWAP_FLAG_PREFER flag is specified in the swapon() swapflags ar‐
23       gument, the new swap area will have a  higher  priority  than  default.
24       The priority is encoded within swapflags as:
25
26           (prio << SWAP_FLAG_PRIO_SHIFT) & SWAP_FLAG_PRIO_MASK
27
28       If  the  SWAP_FLAG_DISCARD  flag is specified in the swapon() swapflags
29       argument, freed swap pages will be discarded before they are reused, if
30       the  swap device supports the discard or trim operation.  (This may im‐
31       prove performance on some Solid State Devices, but often it does  not.)
32       See also NOTES.
33
34       These  functions  may  be used only by a privileged process (one having
35       the CAP_SYS_ADMIN capability).
36
37   Priority
38       Each swap area has a priority, either high or low.  The default  prior‐
39       ity  is low.  Within the low-priority areas, newer areas are even lower
40       priority than older areas.
41
42       All priorities set with swapflags are high-priority,  higher  than  de‐
43       fault.   They  may  have  any  nonnegative  value chosen by the caller.
44       Higher numbers mean higher priority.
45
46       Swap pages are allocated from areas in priority order, highest priority
47       first.   For areas with different priorities, a higher-priority area is
48       exhausted before using a lower-priority area.  If  two  or  more  areas
49       have the same priority, and it is the highest priority available, pages
50       are allocated on a round-robin basis between them.
51
52       As of Linux 1.3.6, the kernel usually follows these  rules,  but  there
53       are exceptions.
54

RETURN VALUE

56       On  success,  zero is returned.  On error, -1 is returned, and errno is
57       set to indicate the error.
58

ERRORS

60       EBUSY  (for swapon()) The specified path is already  being  used  as  a
61              swap area.
62
63       EINVAL The  file  path exists, but refers neither to a regular file nor
64              to a block device;
65
66       EINVAL (swapon()) The indicated path does not contain a valid swap sig‐
67              nature or resides on an in-memory filesystem such as tmpfs(5).
68
69       EINVAL (since Linux 3.4)
70              (swapon()) An invalid flag value was specified in swapflags.
71
72       EINVAL (swapoff()) path is not currently a swap area.
73
74       ENFILE The system-wide limit on the total number of open files has been
75              reached.
76
77       ENOENT The file path does not exist.
78
79       ENOMEM The system has insufficient memory to start swapping.
80
81       EPERM  The caller does not have the CAP_SYS_ADMIN capability.  Alterna‐
82              tively, the maximum number of swap files are already in use; see
83              NOTES below.
84

STANDARDS

86       Linux.
87

HISTORY

89       The swapflags argument was introduced in Linux 1.3.2.
90

NOTES

92       The partition or path must be prepared with mkswap(8).
93
94       There is an upper limit on the number of swap files that may  be  used,
95       defined  by  the  kernel  constant MAX_SWAPFILES.  Before Linux 2.4.10,
96       MAX_SWAPFILES has the value 8; since Linux 2.4.10, it has the value 32.
97       Since  Linux 2.6.18, the limit is decreased by 2 (thus: 30) if the ker‐
98       nel is built with the CONFIG_MIGRATION option (which reserves two  swap
99       table  entries  for  the  page  migration  features of mbind(2) and mi‐
100       grate_pages(2)).  Since Linux 2.6.32, the limit is further decreased by
101       1  if the kernel is built with the CONFIG_MEMORY_FAILURE option.  Since
102       Linux 5.14, the limit is further decreased by 4 if the kernel is  built
103       with the CONFIG_DEVICE_PRIVATE option.
104
105       Discard  of swap pages was introduced in Linux 2.6.29, then made condi‐
106       tional on the SWAP_FLAG_DISCARD flag in Linux 2.6.36, which still  dis‐
107       cards  the  entire swap area when swapon() is called, even if that flag
108       bit is not set.
109