System Administration Guide, Volume 1

The /etc/vfstab File

After the system is installed, swap slices and files are listed in the /etc/vfstab file and are activated by the /sbin/swapadd script when the system is booted.

An entry for a swap device in the /etc/vfstab file contains:

Because the file system containing a swap file must be mounted before the swap file is activated, make sure that the entry that mounts the file system comes before the entry that activates the swap file in the /etc/vfstab file.