1SWAPON(8)                    System Administration                   SWAPON(8)
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NAME

6       swapon, swapoff - enable/disable devices and files for paging and swap‐
7       ping
8

SYNOPSIS

10       swapon [options] [specialfile...]
11       swapoff [-va] [specialfile...]
12

DESCRIPTION

14       swapon is used to specify devices on which paging and swapping  are  to
15       take place.
16
17       The  device or file used is given by the specialfile parameter.  It may
18       be of the form -L label or -U uuid to indicate a  device  by  label  or
19       uuid.
20
21       Calls  to  swapon  normally occur in the system boot scripts making all
22       swap devices available, so that the paging  and  swapping  activity  is
23       interleaved across several devices and files.
24
25       swapoff disables swapping on the specified devices and files.  When the
26       -a flag is given, swapping is disabled on all known  swap  devices  and
27       files (as found in /proc/swaps or /etc/fstab).
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OPTIONS

31       -a, --all
32              All devices marked as ``swap'' in /etc/fstab are made available,
33              except for those with the ``noauto'' option.  Devices  that  are
34              already being used as swap are silently skipped.
35
36       -d, --discard[=policy]
37              Enable  swap  discards,  if the swap backing device supports the
38              discard or trim operation.  This may improve performance on some
39              Solid  State  Devices, but often it does not.  The option allows
40              one to select  between  two  available  swap  discard  policies:
41              --discard=once  to  perform  a single-time discard operation for
42              the whole swap area  at  swapon;  or  --discard=pages  to  asyn‐
43              chronously  discard  freed  swap pages before they are available
44              for reuse.  If no policy is selected, the default behavior is to
45              enable  both  discard  types.  The /etc/fstab mount options dis‐
46              card, discard=once, or discard=pages may also be used to  enable
47              discard flags.
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49       -e, --ifexists
50              Silently  skip  devices that do not exist.  The /etc/fstab mount
51              option nofail may also be used to skip non-existing device.
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53
54       -f, --fixpgsz
55              Reinitialize (exec mkswap) the swap space if its page size  does
56              not  match  that  of the current running kernel.  mkswap(2) ini‐
57              tializes the whole device and does not check for bad blocks.
58
59       -h, --help
60              Display help text and exit.
61
62       -L label
63              Use the partition that has  the  specified  label.   (For  this,
64              access to /proc/partitions is needed.)
65
66       -o, --options opts
67              Specify  swap  options  by  an  fstab-compatible comma-separated
68              string.  For example:
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70                     swapon -o pri=1,discard=pages,nofail /dev/sda2
71
72              The opts string is evaluated last and overrides all  other  com‐
73              mand line options.
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75       -p, --priority priority
76              Specify  the  priority  of the swap device.  priority is a value
77              between -1 and 32767.  Higher numbers indicate higher  priority.
78              See  swapon(2)  for  a full description of swap priorities.  Add
79              pri=value to the option field of /etc/fstab for use with  swapon
80              -a.  When no priority is defined, it defaults to -1.
81
82       -s, --summary
83              Display  swap  usage  summary  by  device.   Equivalent  to "cat
84              /proc/swaps".  This output format is  DEPRECATED  in  favour  of
85              --show that provides better control on output data.
86
87       --show[=column...]
88              Display  a definable table of swap areas.  See the --help output
89              for a list of available columns.
90
91       --output-all
92              Output all available columns.
93
94       --noheadings
95              Do not print headings when displaying --show output.
96
97       --raw  Display --show output without aligning table columns.
98
99       --bytes
100              Display swap size in bytes in --show output instead of in  user-
101              friendly units.
102
103       -U uuid
104              Use the partition that has the specified uuid.
105
106       -v, --verbose
107              Be verbose.
108
109       -V, --version
110              Display version information and exit.
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NOTES

113       You  should  not  use swapon on a file with holes.  This can be seen in
114       the system log as
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116              swapon: swapfile has holes.
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118       The swap file implementation in the kernel expects to be able to  write
119       to  the  file directly, without the assistance of the filesystem.  This
120       is a problem on preallocated files (e.g.  fallocate(1)) on  filesystems
121       like XFS or ext4, and on copy-on-write filesystems like btrfs.
122
123       It  is recommended to use dd(1) and /dev/zero to avoid holes on XFS and
124       ext4.
125
126       swapon may not work correctly when using a swap file with some versions
127       of  btrfs.   This is due to btrfs being a copy-on-write filesystem: the
128       file location may not be  static  and  corruption  can  result.   Btrfs
129       actively disallows the use of swap files on its filesystems by refusing
130       to map the file.
131
132       One possible workaround is to map the swap file to a  loopback  device.
133       This  will  allow  the filesystem to determine the mapping properly but
134       may come with a performance impact.
135
136       Swap over NFS may not work.
137
138       swapon automatically detects and rewrites a swap space  signature  with
139       old software suspend data (e.g. S1SUSPEND, S2SUSPEND, ...). The problem
140       is that if we don't do it, then we get data corruption the next time an
141       attempt at unsuspending is made.
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143

ENVIRONMENT

145       LIBMOUNT_DEBUG=all
146              enables libmount debug output.
147
148       LIBBLKID_DEBUG=all
149              enables libblkid debug output.
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151

SEE ALSO

153       swapoff(2), swapon(2), fstab(5), init(8), mkswap(8), mount(8), rc(8)
154

FILES

156       /dev/sd??  standard paging devices
157       /etc/fstab ascii filesystem description table
158

HISTORY

160       The swapon command appeared in 4.0BSD.
161

AVAILABILITY

163       The  swapon  command is part of the util-linux package and is available
164       from https://www.kernel.org/pub/linux/utils/util-linux/.
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168util-linux                       October 2014                        SWAPON(8)
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