sbin/swapon/swapon.8

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.Dd November 29, 2021
.Dt SWAPON 8
.Os
.Sh NAME
.Nm swapon , swapoff , swapctl
.Nd "specify devices for paging and swapping"
.Sh SYNOPSIS
.Nm swapon
.Oo Fl F Ar fstab
.Oc
.Fl aLq | E Ar
.Nm swapoff
.Oo Fl F Ar fstab
.Oc
.Fl afLq | Ar
.Nm swapctl
.Op Fl AfghklmsU
.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 unlimited.
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
or
.Dq late
option is also set.
If the
.Fl L
option is specified,
swap devices with the
.Dq late
option will be added as well as ones with no option.
If the
.Fl q
option is used,
informational messages will not be
written to standard output when a swap device is added.
The
.Fl E
option causes each of following devices to receive a
.Dv BIO_DELETE
command.
This command marks the device's blocks as unused, except those that
might store a disk label.
This marking can erase a crash dump.
To delay
.Nm swapon
for a device until after
.Nm savecore
has copied the crash dump to another location, use the
.Dq late
option.
.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.
If the
.Fl L
option is specified,
only swap devices with the
.Dq late
option will be removed.
If the
.Fl q
option is used,
informational messages will not be
written to standard output when a swap device is removed.
Note that
.Nm swapoff
will fail and refuse to remove a swap device if a very conservative
check does not conclude that there is sufficient VM (memory +
remaining swap devices) to run the system.
The
.Fl f
option turns off this check, which could deadlock the system
if there is insufficient swap space remaining.
.Pp
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
Other options supported by both
.Nm swapon
and
.Nm swapoff
are as follows:
.Bl -tag -width indent
.It Fl F Ar fstab
Specify the
.Pa fstab
file to use (in conjunction with
.Fl a Ns ).
.El
.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/{ada,da}?s?b" -compact
.It Pa /dev/{ada,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 .