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

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

54       On  success,  zero is returned.  On error, -1 is returned, and errno is
55       set appropriately.
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ERRORS

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

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

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

109       This page is part of release 4.16 of the Linux  man-pages  project.   A
110       description  of  the project, information about reporting bugs, and the
111       latest    version    of    this    page,    can     be     found     at
112       https://www.kernel.org/doc/man-pages/.
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116Linux                             2017-09-15                         SWAPON(2)