HP-UX Reference (11i v3 07/02) - 1M System Administration Commands N-Z (vol 4)
s
swapon(1M) swapon(1M)
NAME
swapon - enable device or file system for paging
SYNOPSIS
Form 1: Enable all defined swap areas
/usr/sbin/swapon -a
[-u][-t
type]...
Form 2: Enable paging on specified block devices (for the current boot)
/usr/sbin/swapon
[-e -f][
-p priority][-u] device ...
Form 3: Define the primary paging device (for subsequent boots)
/usr/sbin/swapon
[-e -f] -s
[-S start][-L length][-u] device
Form 4: Unconfigure a previously set primary paging device (for subsequent boots)
/usr/sbin/swapon -R
device
Form 5: Enable file system swap (preferred form)
/usr/sbin/swapon [
-m min][-l limit][-r reserve][-p priority] directory ...
Form 6: Enable file system swap (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. swapon
com-
mand 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.
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 option is given (see Form 1 of SYNOPSIS), 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(4)), the system determines
which block device or file system to use. The 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.
In Form 2, the -p option enables specific block devices to be used for paging for the current boot. The dev-
ice 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 -s 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 -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 -f 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 swapon determines that the device is being used by the savecrash command to retrieve
436 Hewlett-Packard Company − 1 − HP-UX 11i Version 3: February 2007