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 swapon(1M)							  swapon(1M)




 NAME
      swapon - enable device or file system for paging

 SYNOPSIS
    Preferred Forms:
      /usr/sbin/swapon -a [-u] [-t type]...

      /usr/sbin/swapon [-e |-f] [-p priority] [-u] device ...

      /usr/sbin/swapon [-m min] [-l limit] [-r reserve] [-p priority]
	   directory ...

    Obsolescent Form:
      /usr/sbin/swapon directory [min limit reserve priority]

 DESCRIPTION
      The swapon command enables devices or file systems on which paging is
      to take place.  (NOTE: the term `swap' refers to an obsolete
      implementation of virtual memory; HP-UX actually implements virtual
      memory by way of paging rather than swapping.  This command and others
      retain names derived from `swap' for historical reasons.)

      By enabling a device for paging, the device can be accessed directly
      (without going through the file system) during paging activity.  When
      a file system is enabled for paging, the device(s) on which the file
      system resides are accessed indirectly through the file system.  There
      are advantages and disadvantages to both type of paging.	Keep the
      following tradeoffs in mind when enabling devices or file systems for
      paging.

      Paging directly to a device is significantly faster than doing so
      through the file system.	However, the space on the device that is
      allocated to paging cannot be used for anything else, even if it is
      not being actively used for paging.

      Paging through a file system, while slower, provides a more efficient
      use of the space on the device.  Space that is not being used for
      paging in this case can be used by the file system.  Paging across a
      network to a remote machine is always file system paging.

      The system begins by paging on only a single device so that only one
      disk is required at bootstrap time.  Calls to swapon normally occur in
      the system startup script /sbin/init.d/swap_start making all paging
      space available so that the paging activity is interleaved across
      several disks.

      Normally, the -a argument is given, causing all devices marked as swap
      and all file systems marked as swapfs in the file /etc/fstab to be
      made available to the paging system.  By using the fields in
      /etc/fstab (special_file_name or directory; see fstab(5)), the system
      determines which block device or file system to use.  The



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 swapon(1M)							  swapon(1M)




      special_file_name specified for each swap entry must specify a block
      special file.  The directory specified for each swapfs entry must
      specify a directory within the file system to be enabled.

      The second form of swapon enables individual block devices to be used
      for paging.  The device name must specify a block special file.  If
      more than one device is given, any options specified will be applied
      to all devices.  If a file system exists on the specified block device
      and neither an -e nor -f option is specified, swapon fails and an
      error message is given.  This prevents a file system from being
      inadvertently destroyed.	To request paging in the space between the
      end of the file system and the end of the device, use -e.	 To force
      paging to a device containing a file system (destroying the file
      system), the -f option can be used.  Use this with extreme caution!

      In either of the previous forms, an attempt to enable paging to a
      device will fail and a warning message will be issued if swapon
      determines that the device is being used by the savecore command to
      retrieve system dump information (see savecore(1M)).  The -u option
      can be used to forcibly enable paging to devices being used by
      savecore; however, this may overwrite system dump information
      contained on the device.

      The last two forms of swapon provide two different methods for
      enabling file systems for paging.	 The third form is the preferred
      method, with the fourth being provided only for backward
      compatibility.  The directory name specifies a directory on the file
      system that is to be enabled for paging.	A directory named /paging is
      created at the root of the specified file system (unless the file
      system's name ends with /paging).	 All paging files are created within
      this directory.  The optional arguments to the fourth form have the
      same meaning as the arguments to the options in the third form.  Note
      that, in the fourth form, if any of the optional arguments are
      specified, all must be specified.	 In the third form, if more than one
      directory is given, any options specified will be applied to all
      directories.

      After a file system has been enabled for paging, the optional
      arguments can be modified by subsequent swapon commands.

    Options
      swapon recognizes the following options and arguments:

	   -a		  Cause all devices marked as swap and all file
			  systems marked as swapfs in the file /etc/fstab to
			  be made available to the paging system.  The
			  options field in /etc/fstab entries is read by
			  swapon, and must contain elements formatted as
			  follows:





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			  min=min	 See the -m option for the value of
					 min.

			  lim=limit	 See the -l option for the value of
					 limit.	 (File system paging areas
					 only.)

			  res=reserve	 See the -r option for the value of
					 reserve.  (File system paging areas
					 only.)

			  pri=priority	 See the -p option for the value of
					 priority.  (File system paging
					 areas only.)

			  end		 See the -e option for the meaning
					 of this option.  (Device paging
					 areas only.)

			  See fstab(4) for an example entry.

	   -e		  Use space after the end of the file system on the
			  block device for paging.  An error message is
			  returned if no file system is found on the device.
			  This option cannot be used with the -f option.  Do
			  not confuse this with paging to a file system.
			  This option is for use with a disk that has both a
			  file system and dedicated paging space on it.

	   -f		  Force the device to be enabled, which will destroy
			  the file system on it.  Use with extreme caution.
			  Normally, if a file system exists on the device to
			  be enabled, swapon fails and displays an error
			  message.  This option cannot be used with the -e
			  option.

	   -l limit	  limit specifies the maximum space the paging
			  system is allowed to take from the disk, provided
			  space is available that is not reserved for
			  exclusive use by the file system.  The value of
			  limit is rounded up so that it is a multiple of
			  the paging allocation chunk size, which is set
			  with the kernel tunable parameter swchunk (see
			  config(1M) and swapinfo(1M)).	 See WARNINGS.	The
			  default value for limit is 0, indicating there is
			  no limit to the amount of file system space the
			  paging system can use.

			  limit can be specified in decimal (no prefix),
			  octal (0 prefix), or hexadecimal (0x prefix).	 It
			  may be specified in units of kilobytes (k suffix),



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 swapon(1M)							  swapon(1M)




			  megabytes (M suffix), or file system blocks (no
			  suffix).  (A kilobyte is 1024 bytes; a megabyte is
			  1024 kilobytes; the size of a file system block is
			  determined by the administrator when the file
			  system is created.)

	   -m min	  min indicates the space the paging system will
			  initially take from the file system.	The value of
			  min is rounded up so that it is a multiple of the
			  paging allocation chunk size, which is set with
			  the kernel tunable parameter swchunk (see
			  config(1M) and swapinfo(1M)).	 The default value
			  for min is 0, indicating no paging space is to be
			  allocated initially.	min can be specified in the
			  same forms as limit, above.

	   -p priority	  priority indicates the order in which space is
			  taken from the file systems and devices used for
			  paging.  Space is taken from the systems with
			  lower priority numbers first.	 Under most
			  circumstances, space is taken from device paging
			  areas before file system paging areas, regardless
			  of priority.	See "Paging Allocation" in
			  swapinfo(1M) for more information.  priority can
			  have a value from 0 to 10 and has a default value
			  of 1.

	   -r reserve	  reserve specifies the space, in addition to the
			  space currently occupied by the file system, that
			  is reserved for file system use only, making it
			  unavailable to the paging system.  This reserved
			  space is in addition to the minimum free space
			  specified by the administrator when the file
			  system was created.  See WARNINGS.  The default
			  value for reserve is 0 indicating that no file
			  system space is reserved for file system use only.
			  reserve can be specified in the same forms as
			  limit, above.

	   -t type	  Restrict the type of the paging area.	 If the -t
			  option is omitted, all of the paging areas defined
			  in /etc/fstab are made available.  type can have
			  one of the following values:

			  dev	    Device paging areas.

			  fs	    File system paging areas.

			  local	    Paging areas defined on the local
				    system.




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			  remote    Paging areas defined on remote systems.

	   -u		  Unlock block device files which are being used by
			  the savecore command.	 Normally, swapon will not
			  enable paging on a device if it is being used by
			  savecore to retrieve system dump information.	 The
			  list of devices in use is maintained in the file
			  /etc/savecore.LCK.  This option forces the device
			  to be enabled, which may overwrite any system dump
			  information contained on the device.	This option
			  should be used with extreme caution.

 RETURN VALUE
      swapon returns one of the following values:

	    0	Successful completion.
	   >&gt&gt>0	An error condition occurred.

 EXAMPLES
      The first two examples enable paging to the file system containing the
      /paging directory.  The maximum number of file system blocks available
      to the paging system is set to 5000, the number of file system blocks
      reserved for file system use only is set to 10000, and the priority is
      set to 2.	 The number of file system blocks initially taken by the
      paging system defaults to 0 in the first example, and is set to 0 in
      the second example.  On a file system with the default 8kB block size,
      these examples allocate approximately 40MB of file system paging.

	   /usr/sbin/swapon -l 5000 -r 10000 -p 2 /paging
	   /usr/sbin/swapon /paging 0 5000 10000 2

      This example enables paging to two block devices and sets the priority
      of both devices to 0.

	   /usr/sbin/swapon -p 0 /dev/dsk/c10t0d0 /dev/dsk/c13t0d0

      This example enables paging to a block device, using the space after
      the end of the file system for paging and letting the priority default
      to 1.

	   /usr/sbin/swapon -e /dev/dsk/c4t0d0

      This example enables paging to a block device, forcing paging even if
      a file system exists on the device.

	   /usr/sbin/swapon -f /dev/dsk/c12t0d0

 WARNINGS
      Once file system blocks have been allocated for paging space, the file
      system cannot be unmounted unless the system is rebooted.




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      If any paging area becomes unavailable while the system is running,
      for example if a network failure occurs while paging to a remote
      system, the system will immediately halt.

      The file system block size used by the -l, -m, and -r options varies
      between file systems, and is defined by the system administrator at
      the time the file system is created.  The dumpfs command can be used
      to determine the block size for a particular file system (see
      dumpfs(1M)).

      When using the -l and -r options, the reserve space specified by the
      -r option takes precedence over the -l option.  Thus, if:

	   D	     = Total disk space available to ordinary users
	   R	     = Reserve space specified by the -r option
	   limit     = Paging space limit specified by the -l option
	   L	     = Space currently available to the paging system
	   F	     = Space currently occupied by the file system

      the following relationships hold:

	   F + R + limit < D   In normal operation

	   L = 0	       If F + R >= D

	   0 <= L <= limit     If F + R + limit >= D

 FILES
      /dev/dsk/ccardttargetddevice	 Normal paging devices
      /etc/fstab			 File system table
      /etc/savecore.LCK			 List of devices being used by
					 savecore

 AUTHOR
      swapon was developed by HP and the University of California, Berkeley.

 SEE ALSO
      config(1M), savecore(1M), swapinfo(1M), swapon(2), fstab(4).
















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