swapon - enable device or file system for paging
Form 1: Enable all defined swap areas
Form 2: Enable paging on specified block devices (for the current boot)
priority] device ...
Form 3: Define the primary paging device (for subsequent boots)
start] length] device
Form 4: Unconfigure a previously set primary paging device (for subsequent boots)
Form 5: Enable file system swap (preferred form)
min] limit] reserve] priority] directory ...
Form 6: Enable file system swap (obsolescent form)
directory [min limit reserve priority]
The command enables devices or file systems on which paging is to take place. command also configures primary paging device for next boot.
(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 directly to a device is significantly faster than doing so through the file system. However, the space on the device that is allo-
cated 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 normally occur in the
system startup script making all paging space available so that the paging activity is interleaved across several disks.
Normally, the option is given (see Form 1 of causing all devices marked as and all file systems marked as in the file to be made available
to the paging system. By using the fields in (special_file_name or directory; see fstab(4)), the system determines which block device or
file system to use. The special_file_name specified for each entry must specify a block special file. The directory specified for each
entry must specify a directory within the file system to be enabled.
In Form 2, the option enables specific block devices to be used for paging for the current boot. The device arguments must specify block
special files. If more than one device is given, any options specified will be applied to all devices.
In Form 3, the option configures the block device to be used as the primary paging area for subsequent boots.
In either Form 2 or Form 3, if a file system exists on the specified block device and neither an nor option is specified, 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 To force paging to a device containing a file system (destroying the file system), the
option can be used. Use with extreme caution!
In either Form 2 or Form 3, an attempt to enable paging to a device will fail and a warning message will be issued if determines that the
device is being used by the command to retrieve system dump information (see savecrash(1M)). The option can be used to forcibly enable
paging to devices being used by the command; however, this may overwrite system dump information contained on the device.
In Form 4, the option unconfigures the block device that was previously defined as the primary paging area for subsequent boots (see
The last two forms of provide methods for enabling file systems for paging. Form 5 is the preferred method. Form 6 is obsolescent and
provided only for backward compatibility. The directory name in these forms specifies a directory on the file system that is to be enabled
for paging. A directory named is created at the root of the specified file system (unless the file system's name ends with All paging
files are created within this directory. The optional arguments to the sixth form have the same meaning as the arguments to the options in
Form 5. Note that, in Form 6, if any of the optional arguments are specified, all must be specified. In Form 5, if more than one direc-
tory 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 commands.
recognizes the following options and arguments:
Cause all devices marked as
and all file systems marked as in the file to be made available to the paging system. The options field in entries
is read by and must contain elements formatted as follows:
See the option for the value of min.
See the option for the value of limit. (File system paging areas only.)
See the option for the value of reserve. (File system paging areas only.)
See the option for the value of priority. (File system paging areas only.)
See the option for the meaning of this option. (Device paging areas only.)
See fstab(4) for an example entry.
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 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.
Force the device to be enabled, which will destroy the file system on it. Use with extreme caution. Normally, if a file sys-
tem exists on the device to be enabled, fails and displays an error message. This option cannot be used with the
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 (see swchunk(5), kctune(1M), and swap-
info(1M)). See 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 prefix), or hexadecimal prefix). It may be specified in units
of kilobytes suffix), megabytes 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.)
When configuring the primary paging device for next boot,
length specifies the maximum number of blocks that will be used for paging. The default for length is to the end-of-
device. can only be specified when defining primary swap space for subsequent boots; therefore, must be used in con-
junction with the option.
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 (see
swchunk(5), kctune(1M), and swapinfo(1M)). The default value for min is 0, indicating no paging space is to be allo-
cated initially. min can be specified in the same forms as limit, above.
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 informa-
tion. priority can have a value from 0 to 10 and has a default value of 1.
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 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,
Unconfigure the primary paging device that was previously set
(with the option) as the primary paging area for subsequent boots.
Configure the primary paging device for the next
and subsequent boots. See also the and options.
When configuring the primary paging device for subsequent boots,
start specifies the block address on the device where the paging area will begin. The default value for start is 0
indicating that the device is dedicated to paging. A starting block can only be specified when defining primary swap
space for subsequent boots; therefore, must be used in conjunction with the option.
Restrict the type of the paging area.
If the option is omitted, all of the paging areas defined in are made available. type can have one of the following
Device paging areas.
File system paging areas.
Paging areas defined on the local system.
Paging areas defined on remote systems.
Unlock block device files which are being used by the
command. Normally, will not enable paging on a device if it is being used by command to retrieve system dump infor-
mation. The list of devices in use is maintained in the file 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.
returns one of the following values:
An error condition occurred.
The first two examples enable paging to the file system containing the 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.
This example enables paging to two block devices and sets the priority of both devices to 0.
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
This example enables paging to a block device, forcing paging even if a file system exists on the device.
This example defines the primary paging device for the next boot, using the space after the end of the file system to the end of the device
This example defines the primary paging device for the next boot, using 8192Kb of the device for paging, starting 1024Kb from the start of
On systems running VxVM 3.5, the swap volumes to be configured for system crash dumps should be created with the usage type as during the
creation of the swap volume. Not doing so will cause dump corruption. You could use the option of vxassist(1M) to do the same.
Once file system blocks have been allocated for paging space, the file system cannot be unmounted unless the system is rebooted.
If any paging area becomes unavailable while the system is running, for example if a network failure occurs while paging to a remote sys-
tem, the system will immediately halt.
The file system block size used by the and options varies between file systems, and is defined by the system administrator at the time the
file system is created. The command can be used to determine the block size for a particular file system (see dumpfs(1M)).
When using the and options, the reserve space specified by the option takes precedence over the option. Thus, if:
D = Total disk space available to ordinary users
R = Reserve space specified by the option
limit = Paging space limit specified by the 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
Normal paging devices
File system table
List of devices being used by command
was developed by HP and the University of California, Berkeley.
kctune(1M), savecrash(1M), swapinfo(1M), vxassist(1M), swapctl(2), swapon(2), fstab(4), swchunk(5).