swapon(1M)swapon(1M)NAMEswapon - enable device or file system for paging
SYNOPSIS
Form 1: Enable all defined swap areas
type]...
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)
device
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]
DESCRIPTION
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 sys‐
tem resides are accessed indirectly through the file system. There are
advantages and disadvantages to both type of paging. Keep the follow‐
ing 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 pag‐
ing in this case can be used by the file system. Paging across a net‐
work 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 direc‐
tory; 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 con‐
taining 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 sys‐
tem 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 option).
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 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 commands.
Options
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 ele‐
ments 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 sys‐
tem and dedicated paging space on it.
Force the device to be enabled, which will destroy the file
system on it. Use with extreme caution. Nor‐
mally, if a file system exists on the device to
be enabled, fails and displays an error message.
This option cannot be used with the option.
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 ker‐
nel tunable parameter (see swchunk(5),
kctune(1M), and swapinfo(1M)). See The default
value for limit is 0, indicating there is no
limit to the amount of file system space the pag‐
ing 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 suf‐
fix). (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 ini‐
tially 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
allocated 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 prior‐
ity 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.
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 sys‐
tem 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,
above.
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 values:
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 information. 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.
RETURN VALUE
returns one of the following values:
Successful completion.
An error condition occurred.
EXAMPLES
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 exam‐
ple. 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
to 1.
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 for
paging.
This example defines the primary paging device for the next boot, using
8192Kb of the device for paging, starting 1024Kb from the start of the
device.
WARNINGS
On systems running VxVM 3.5, the swap volumes to be configured for sys‐
tem 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 system,
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
FILES
Normal paging devices
File system table
List of devices being used by command
AUTHOR
was developed by HP and the University of California, Berkeley.
SEE ALSOkctune(1M), savecrash(1M), swapinfo(1M), vxassist(1M), swapctl(2),
swapon(2), fstab(4), swchunk(5).
swapon(1M)