swapon man page on Archlinux

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SWAPON(8)		     System Administration		     SWAPON(8)

       swapon, swapoff - enable/disable devices and files for paging and swap‐

       swapon [ options ] [specialfile...]
       swapoff [-va] [specialfile...]

       swapon is used to specify devices on which paging and swapping  are  to
       take place.

       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

       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.

       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).

       -a, --all
	      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.

       -d, --discard [=policy]
	      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:
	      --discard=once  to  perform  a single-time discard operation for
	      the whole swap area at swapon;  or  --discard=pages  to  discard
	      freed  swap pages before they are reused, while swapping.	 If no
	      policy is selected, the default behavior is to enable both  dis‐
	      card types.  The /etc/fstab mount options discard, discard=once,
	      or discard=pages may be also used to enable discard flags.

       -e, --ifexists
	      Silently skip devices that do not exist.	The  /etc/fstab	 mount
	      option nofail may be also used to skip non-existing device.

       -f, --fixpgsz
	      Reinitialize (exec /sbin/mkswap) the swap space if its page size
	      does not match that of the current  running  kernel.   mkswap(2)
	      initializes the whole device and does not check for bad blocks.

       -h, --help
	      Display help text and exit.

       -L label
	      Use  the	partition  that	 has  the specified label.  (For this,
	      access to /proc/partitions is needed.)

       -p, --priority priority
	      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 priority is not defined it defaults to -1.

       -s, --summary
	      Display  swap  usage  summary  by	 device.  Equivalent  to  "cat
	      /proc/swaps".  Not available before Linux 2.1.25.

       --show [column, ...]
	      Display definable device table similar to --summary output.  See
	      --help output for column list.

	      Do not print headings when displaying --show output.

       --raw  Display --show output without aligning table columns.

	      Display swap size in bytes in  --show  output  instead  of  user
	      friendly	size and unit.	-U uuid Use the partition that has the
	      specified uuid.

       -v, --verbose
	      Be verbose.

       -V, --version
	      Display version information and exit.

       You should not use swapon on a file with holes.	Swap over NFS may  not

       swapon automatically detects and rewrites 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.

       swapon may not work correctly when using a swap file with some versions
       of  btrfs.   This  is due to the swap file implementation in the kernel
       expecting to be able to write to the file directly, without the	assis‐
       tance  of the file system.  Since btrfs is a copy-on-write file system,
       the file location may not be static and corruption  can	result.	 Btrfs
       actively	 disallows the use of files on its file systems by refusing to
       map the file. This can be seen in the system log as  "swapon:  swapfile
       has  holes."  One  possible workaround is to map the file to a loopback
       device. This will allow the file system to determine the mapping	 prop‐
       erly but may come with a performance impact.

	      enables debug output.

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

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

       The swapon command appeared in 4.0BSD.

       The  swapon  command is part of the util-linux package and is available
       from ftp://ftp.kernel.org/pub/linux/utils/util-linux/.

util-linux			September 1995			     SWAPON(8)

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