How do I add swap on FreeBSD version 9 or older?

You will create the swap file by typing the following dd command as the root user:

dd if=/dev/zero of=/root/swap.8G.bin bs=1M count=8192

This should create an 8GB file called swap.8G.bin in /root/. To make sure this worked you can type:

ls -alh  /root/swap.8G.bin

For security reason set the permissions, run:

chmod 0600 /root/swap.8G.bin
ls -alh  /root/swap.8G.bin

Sample outputs:

How do I activate swap space on the boot time?

To add this to your rc.conf you will type:

echo 'swapfile="/root/swap.8G.bin"' >> /etc/rc.conf

If you want to see if it is there in your rc.conf you can type:

tail /etc/rc.conf

Reboot the system:

reboot

A note about enabling the swap file immediately without rebooting the system

If you want to apply the swapfile immediately type the following command:

## Enable swap space ## 
mdconfig -a -t vnode -f /root/swap.8G.bin -u 0 
 
## Find out configured devices i.e. swap device name ##
mdconfig -l -v
 
## Turn it on ##
swapon /dev/md0

Sample outputs:

swapinfo -k
swapinfo -k | grep '/root/swap.8G.bin'
swapinfo -h

Sample outputs:

Device          1K-blocks     Used    Avail Capacity
/dev/ada0p3       1048540     736K     1.0G     0%
/dev/md0          8388608       0B     8.0G     0%
Total             9437148     736K     9.0G     0%

How to set up swap file on FreeBSD version 10.x or later

First, create the swap file (128M) using dd command:

dd if=/dev/zero of=/root/swap1 bs=1m count=128

Set the proper permissions on the new file for security reason:

chmod 0600 /root/swap1

Edit /etc/fstab, enter:

vi /etc/fstab

Add/append the following line:

## md42 will be assigned by system, use any unused device name (run 'mdconfig -lv' to get list of attached memory device names) ##
md42	none	swap	sw,file=/root/swap1	0	0

If you want to see if it is there in your /etc/fstab you can type:

tail /etc/fstab

Now, swap space will be added on system boot time. To add and activate swap space immediately, run:

swapon -aq

To see details of your swap type:

swapinfo -k

Sample session from my FreeBSD10 based server:

A note about securing and encrypting swap space on a FreeBSD server

Encrypting swap space can avoid leakage of sensitive information such as passwords and other data in memory.

Procedure to encrypt swap file

Type the following command to create a swap file called /root/en.swap0
# dd if=/dev/random of=/root/en.swap0 bs=1m count=64
# mdconfig -a -t vnode -f /root/en.swap0
# geom eli init md0
Sample outputs:

Enter new passphrase:
Reenter new passphrase: 
 
Metadata backup can be found in /var/backups/md0.eli and
can be restored with the following command:
 
	# geli restore /var/backups/md0.eli md0

Attach md0, enter:
# geom eli attach md0
Turn on encrpted swap file:
# swapon /dev/md0.eli
Verify newly created swap space:
# swapinfo -k
Sample session:

How can I disable devices and files for paging and swapping on FreeBSD?

Type the following command to disable /dev/md0 swap space:
# swapoff /dev/md0
# swpainfo -k

How can I display swap usage summary on FreeBSD?

Use the top command:
# top
Sample outputs (look for Swap in outputs):

last pid:   874;  load averages:  0.47,  0.32,  0.27                                                                                                                                                                                                    up 0+00:34:48  16:52:35
22 processes:  1 running, 21 sleeping
CPU:  0.0% user,  0.0% nice,  0.0% system,  0.0% interrupt,  100% idle
Mem: 14M Active, 13M Inact, 104M Wired, 80M Buf, 1841M Free
Swap: 1216M Total, 1216M Free
 
  PID USERNAME    THR PRI NICE   SIZE    RES STATE    TIME    WCPU COMMAND
  721 root          1  20    0 25328K  3704K select   0:00   0.00% ntpd
  755 root          1  20    0 86084K  6896K select   0:00   0.00% sshd
  765 root          1  20    0 23980K  5188K select   0:00   0.00% sendmail
  758 root          1  20    0 23492K  3452K pause    0:00   0.00% csh
....
..

You can also use pstat or swapinfo commands:
# pstat -s
OR
# swapinfo -k
You can also use vmstat/systat commands:
# vmstat
# systat swap
See man pages for more info:
$ man vmstat
$ man systat
$ man top
$ man swapinfo
$ man pstat