FreeBSD Handbook
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6.14 Adding Swap Space

No matter how well you plan, sometimes a system does not run as you expect. If you find you need more swap space, it is simple enough to add. You have three ways to increase swap space: adding a new hard drive, enabling swap over NFS, and creating a swap file on an existing partition.

6.14.1 Swap on a New Hard Drive

The best way to add swap, of course, is to use this as an excuse to add another hard drive. You can always use another hard drive, after all. If you can do this, go reread the discussion of swap space from the Initial Configuration section of the Handbook for some suggestions on how to best arrange your swap.

6.14.2 Swapping over NFS

Swapping over NFS is only recommended if you do not have a local hard disk to swap to. Swapping over NFS is slow and inefficient in versions of FreeBSD prior to 4.X. It is reasonably fast and efficient in 4.0-RELEASE and newer. Even with newer versions of FreeBSD, NFS swapping will be limited by the available network bandwidth and puts an additional burden on the NFS server.

6.14.3 Swapfiles

You can create a file of a specified size to use as a swap file. In our example here we will use a 64MB file called /usr/swap0. You can use any name you want, of course.

Example 6-1. Creating a Swapfile on FreeBSD 4.X

Be certain that your kernel configuration includes the vnode driver. It is not in recent versions of GENERIC.
```
pseudo-device   vn 1   #Vnode driver (turns a file into a device)
```
create a vn-device:
```
# cd /dev
# sh MAKEDEV vn0
```

create a swapfile (/usr/swap0):

# dd if=/dev/zero of=/usr/swap0 bs=1024k count=64

set proper permissions on (/usr/swap0):
```
# chmod 0600 /usr/swap0
```

enable the swap file in /etc/rc.conf:

swapfile="/usr/swap0"   # Set to name of swapfile if aux swapfile desired.

Reboot the machine or to enable the swap file immediately, type:
```
# vnconfig -e /dev/vn0b /usr/swap0 swap
```

Example 6-2. Creating a Swapfile on FreeBSD 5.X

Be certain that your kernel configuration includes the memory disk driver (md(4)). It is default in GENERIC kernel.
```
device   md   # Memory "disks"
```

create a swapfile (/usr/swap0):

# dd if=/dev/zero of=/usr/swap0 bs=1024k count=64

set proper permissions on (/usr/swap0):
```
# chmod 0600 /usr/swap0
```

enable the swap file in /etc/rc.conf:

swapfile="/usr/swap0"   # Set to name of swapfile if aux swapfile desired.

Reboot the machine or to enable the swap file immediately, type:
```
# mdconfig -a -t vnode -f /usr/swap0 -u 0 && swapon /dev/md0
```

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