12.15 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.

For information on how to encrypt swap space, what options for this task exist and why it should be done, please refer to Section 19.17 of the Handbook.

12.15.1 Swap on a New or Existing Hard Drive

Adding a new hard drive for swap gives better performance than adding a partition on an existing drive. Setting up partitions and hard drives is explained in Section 19.3. Section 12.2 discusses partition layouts and swap partition size considerations.

Use swapon(8) to add a swap partition to the system. For example:

# swapon/dev/ada1s1b

Warning: It is possible to use any partition not currently mounted, even if it already contains data. Using swapon(8) on a partition that contains data will overwrite and destroy that data. Make sure that the partition to be added as swap is really the intended partition before running swapon(8).

To automatically add this swap partition on boot, add an entry to /etc/fstab for the partition:

/dev/ada1s1b	none	swap	sw	0	0

See fstab(5) for an explanation of the entries in /etc/fstab.

12.15.2 Swapping over NFS

Swapping over NFS is only recommended if you do not have a local hard disk to swap to; NFS swapping will be limited by the available network bandwidth and puts an additional burden on the NFS server.

12.15.3 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 12-1. Creating a Swapfile on FreeBSD

  1. The GENERIC kernel already includes the memory disk driver (md(4)) required for this operation. When building a custom kernel, make sure to include the following line in your custom configuration file:

    device   md

    For information on building your own kernel, please refer to Chapter 9.

  2. Create a swapfile (/usr/swap0):

    # dd if=/dev/zero of=/usr/swap0 bs=1024k count=64
  3. Set proper permissions on (/usr/swap0):

    # chmod 0600 /usr/swap0
  4. Enable the swap file in /etc/rc.conf:

    swapfile="/usr/swap0"   # Set to name of swapfile if aux swapfile desired.
  5. Reboot the machine or to enable the swap file immediately, type:

    # mdconfig -a -t vnode -f /usr/swap0 -u 0 && swapon /dev/md0