sys-utils/swapon.8.adoc

   1 //po4a: entry man manual
   2 = swapon(8)
   3 :doctype: manpage
   4 :man manual: System Administration
   5 :man source: util-linux {release-version}
   6 :page-layout: base
   7 :command: swapon
   8
   9 == NAME
  10
  11 swapon, swapoff - enable/disable devices and files for paging and swapping
  12
  13 == SYNOPSIS
  14
  15 *swapon* [options] [_specialfile_...]
  16
  17 *swapoff* [*-va*] [_specialfile_...]
  18
  19 == DESCRIPTION
  20
  21 *swapon* is used to specify devices on which paging and swapping are to take place.
  22
  23 The device or file used is given by the _specialfile_ parameter. It may be of the form *-L* _label_ or *-U* _uuid_ to indicate a device by label or uuid.
  24
  25 Calls to *swapon* normally occur in the system boot scripts making all swap devices available, so that the paging and swapping activity is interleaved across several devices and files.
  26
  27 *swapoff* disables swapping on the specified devices and files. When the *-a* flag is given, swapping is disabled on all known swap devices and files (as found in _/proc/swaps_ or _/etc/fstab_).
  28
  29 == OPTIONS
  30
  31 *-a*, *--all*::
  32 All devices marked as "swap" in _/etc/fstab_ are made available, except for those with the "noauto" option. Devices that are already being used as swap are silently skipped.
  33
  34 *-d*, *--discard*[**=**__policy__]::
  35 Enable swap discards, if the swap backing device supports the discard or trim operation. This may improve performance on some Solid State Devices, but often it does not. The option allows one to select between two available swap discard policies:
  36
  37 *--discard=once*;;
  38 to perform a single-time discard operation for the whole swap area at swapon; or
  39
  40 *--discard=pages*;;
  41 to asynchronously discard freed swap pages before they are available for reuse.
  42
  43 +
  44 If no policy is selected, the default behavior is to enable both discard types. The _/etc/fstab_ mount options *discard*, *discard=once*, or *discard=pages* may also be used to enable discard flags.
  45
  46 *-e*, *--ifexists*::
  47 Silently skip devices that do not exist. The _/etc/fstab_ mount option *nofail* may also be used to skip non-existing device.
  48
  49 *-f*, *--fixpgsz*::
  50 Reinitialize (exec mkswap) the swap space if its page size does not match that of the current running kernel. *mkswap*(8) initializes the whole device and does not check for bad blocks.
  51
  52 *-h*, *--help*::
  53 Display help text and exit.
  54
  55 *-L* _label_::
  56 Use the partition that has the specified _label_. (For this, access to _/proc/partitions_ is needed.)
  57
  58 *-o*, *--options* _opts_::
  59 Specify swap options by an fstab-compatible comma-separated string. For example:
  60 +
  61 *swapon -o pri=1,discard=pages,nofail /dev/sda2*
  62 +
  63 The _opts_ string is evaluated last and overrides all other command line options.
  64
  65 *-p*, *--priority* _priority_::
  66 Specify the priority of the swap device. _priority_ is a value between -1 and 32767. Higher numbers indicate higher priority. See *swapon*(2) for a full description of swap priorities. Add **pri=**__value__ to the option field of _/etc/fstab_ for use with *swapon -a*. When no priority is defined, it defaults to -1.
  67
  68 *-s*, *--summary*::
  69 Display swap usage summary by device. Equivalent to *cat /proc/swaps*. This output format is DEPRECATED in favour of *--show* that provides better control on output data.
  70
  71 *--show*[**=**__column__...]::
  72 Display a definable table of swap areas. See the *--help* output for a list of available columns.
  73
  74 *--output-all*::
  75 Output all available columns.
  76
  77 *--noheadings*::
  78 Do not print headings when displaying *--show* output.
  79
  80 *--raw*::
  81 Display *--show* output without aligning table columns.
  82
  83 *--bytes*::
  84 Display swap size in bytes in *--show* output instead of in user-friendly units.
  85
  86 *-U* _uuid_::
  87 Use the partition that has the specified _uuid_.
  88
  89 *-v*, *--verbose*::
  90 Be verbose.
  91
  92 *-V*, *--version*::
  93 Display version information and exit.
  94
  95 == EXIT STATUS
  96
  97 *swapoff* has the following exit status values since v2.36:
  98
  99 *0*::
 100 success
 101
 102 *2*::
 103 system has insufficient memory to stop swapping (OOM)
 104
 105 *4*::
 106 swapoff syscall failed for another reason
 107
 108 *8*::
 109 non-swapoff syscall system error (out of memory, ...)
 110
 111 *16*::
 112 usage or syntax error
 113
 114 *32*::
 115 all swapoff failed on *--all*
 116
 117 *64*::
 118 some swapoff succeeded on *--all*
 119
 120 The command *swapoff --all* returns 0 (all succeeded), 32 (all failed), or 64 (some failed, some succeeded).
 121 +
 122 The old versions before v2.36 has no documented exit status, 0 means success in all versions.
 123
 124 == ENVIRONMENT
 125
 126 LIBMOUNT_DEBUG=all::
 127 enables *libmount* debug output.
 128
 129 LIBBLKID_DEBUG=all::
 130 enables *libblkid* debug output.
 131
 132 == FILES
 133
 134 _/dev/sd??_::
 135 standard paging devices
 136
 137 _/etc/fstab_::
 138 ascii filesystem description table
 139
 140 == NOTES
 141
 142 === Files with holes
 143
 144 The swap file implementation in the kernel expects to be able to write to the file directly, without the assistance of the filesystem. This is a problem on files with holes or on copy-on-write files on filesystems like Btrfs.
 145
 146 Commands like *cp*(1) or *truncate*(1) create files with holes. These files will be rejected by *swapon*.
 147
 148 Preallocated files created by *fallocate*(1) may be interpreted as files with holes too depending of the filesystem. Preallocated swap files are supported on XFS since Linux 4.18.
 149
 150 The most portable solution to create a swap file is to use *dd*(1) and _/dev/zero_.
 151
 152 === Btrfs
 153
 154 Swap files on Btrfs are supported since Linux 5.0 on files with nocow attribute. See the *btrfs*(5) manual page for more details.
 155
 156 === NFS
 157
 158 Swap over *NFS* may not work.
 159
 160 === Suspend
 161
 162 *swapon* automatically detects and rewrites a swap space signature with old software suspend data (e.g., S1SUSPEND, S2SUSPEND, ...). The problem is that if we don't do it, then we get data corruption the next time an attempt at unsuspending is made.
 163
 164 == HISTORY
 165
 166 The *swapon* command appeared in 4.0BSD.
 167
 168 == SEE ALSO
 169
 170 *swapoff*(2),
 171 *swapon*(2),
 172 *fstab*(5),
 173 *init*(8),
 174 *fallocate*(1),
 175 *mkswap*(8),
 176 *mount*(8),
 177 *rc*(8)
 178
 179 include::../man-common/bugreports.adoc[]
 180
 181 include::../man-common/footer.adoc[]
 182
 183 ifdef::translation[]
 184 include::../man-common/translation.adoc[]
 185 endif::[]