swapon(2)

1SWAPON(2)                  Linux Programmer's Manual                 SWAPON(2)
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NAME

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

9       #include <sys/swap.h>
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11       int swapon(const char *path, int swapflags);
12       int swapoff(const char *path);
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DESCRIPTION

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

53       On  success,  zero is returned.  On error, -1 is returned, and errno is
54       set to indicate the error.
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ERRORS

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

83       These functions are Linux-specific and should not be used  in  programs
84       intended  to be portable.  The second swapflags argument was introduced
85       in Linux 1.3.2.
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NOTES

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

108       This page is part of release 5.12 of the Linux  man-pages  project.   A
109       description  of  the project, information about reporting bugs, and the
110       latest    version    of    this    page,    can     be     found     at
111       https://www.kernel.org/doc/man-pages/.
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115Linux                             2021-03-22                         SWAPON(2)