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/.

util-linux                                       September 1995                                        SWAPON(8)