NAME

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

SYNOPSIS

       Get info:
            swapon -s [-h] [-V]

       Enable/disable:
            swapon [-d] [-f] [-p priority] [-v] specialfile...
            swapoff [-v] specialfile...

       Enable/disable all:
            swapon -a [-e] [-f] [-v]
            swapoff -a [-v]

DESCRIPTION

       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 uuid.

       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
              Discard freed swap pages before they are reused, if the swap  device  supports  the
              discard  or  trim  operation.   This  may  improve  performance on some Solid State
              Devices, but often it does not.  The /etc/fstab mount option discard  may  be  also
              used to enable discard flag.

       -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 the current running kernel.  mkswap(2) initializes the whole device and
              does not check for bad blocks.

       -h, --help
              Provide help.

       -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 0 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.

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

       -U uuid
              Use the partition that has the specified uuid.

       -v, --verbose
              Be verbose.

       -V, --version
              Display version.

NOTES

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

       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 assistance 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
       properly but may come with a performance impact.

SEE ALSO

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

FILES

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

HISTORY

       The swapon command appeared in 4.0BSD.

AVAILABILITY

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