freebsd/sbin/swapon/swapon.8

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.\"     @(#)swapon.8	8.1 (Berkeley) 6/5/93
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.Dd April 29, 2008
.Dt SWAPON 8
.Os
.Sh NAME
.Nm swapon , swapoff , swapctl
.Nd "specify devices for paging and swapping"
.Sh SYNOPSIS
.Nm swapon Fl a | Ar
.Nm swapoff Fl a | Ar
.Nm swapctl
.Op Fl AghklmsU
.Oo
.Fl a Ar
|
.Fl d Ar
.Oc
.Sh DESCRIPTION
The
.Nm swapon , swapoff
and
.Nm swapctl
utilities are used to control swap devices in the system.
At boot time all swap entries in
.Pa /etc/fstab
are added automatically when the system goes multi-user.
Swap devices use a fixed interleave; the maximum number of devices
is specified by the kernel configuration option
.Dv NSWAPDEV ,
which is typically set to 4.
There is no priority mechanism.
.Pp
The
.Nm swapon
utility adds the specified swap devices to the system.
If the
.Fl a
option is used, all swap devices in
.Pa /etc/fstab
will be added, unless their
.Dq noauto
option is also set.
.Pp
The
.Nm swapoff
utility removes the specified swap devices from the system.
If the
.Fl a
option is used, all swap devices in
.Pa /etc/fstab
will be removed, unless their
.Dq noauto
option is also set.
Note that
.Nm swapoff
will fail and refuse to remove a swap device if there is insufficient
VM (memory + remaining swap devices) to run the system.
The
.Nm swapoff
utility
must move swapped pages out of the device being removed which could
lead to high system loads for a period of time, depending on how
much data has been swapped out to that device.
.Pp
The
.Nm swapctl
utility exists primarily for those familiar with other
.Bx Ns s
and may be
used to add, remove, or list swap devices.
Note that the
.Fl a
option is used differently in
.Nm swapctl
and indicates that a specific list of devices should be added.
The
.Fl d
option indicates that a specific list should be removed.
The
.Fl A
and
.Fl U
options to
.Nm swapctl
operate on all swap entries in
.Pa /etc/fstab
which do not have their
.Dq noauto
option set.
.Pp
Swap information can be generated using the
.Xr swapinfo 8
utility,
.Nm pstat
.Fl s ,
or
.Nm swapctl
.Fl l .
The
.Nm swapctl
utility has the following options for listing swap:
.Bl -tag -width indent
.It Fl h
Output values in human-readable form.
.It Fl g
Output values in gigabytes.
.It Fl k
Output values in kilobytes.
.It Fl m
Output values in megabytes.
.It Fl l
List the devices making up system swap.
.It Fl s
Print a summary line for system swap.
.Pp
The
.Ev BLOCKSIZE
environment variable is used if not specifically
overridden.
512 byte blocks are used by default.
.El
.Sh FILES
.Bl -tag -width ".Pa /dev/{ad,da}?s?b" -compact
.It Pa /dev/{ad,da}?s?b
standard paging devices
.It Pa /dev/md?
memory disk devices
.It Pa /etc/fstab
.Tn ASCII
file system description table
.El
.Sh DIAGNOSTICS
These utilities may fail for the reasons described in
.Xr swapon 2 .
.Sh SEE ALSO
.Xr swapon 2 ,
.Xr fstab 5 ,
.Xr init 8 ,
.Xr mdconfig 8 ,
.Xr pstat 8 ,
.Xr rc 8
.Sh HISTORY
The
.Nm swapon
utility appeared in
.Bx 4.0 .
The
.Nm swapoff
and
.Nm swapctl
utilities appeared in
.Fx 5.1 .