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要添加swap分区，FreeBSD和别的linux服务器命令有些不同，经过尝试，以下步骤在FreeBSD11.1上亲测可用。
1. Create the swap file, /usr/swap0:
# dd if=/dev/zero of=/usr/swap0 bs=1024k count=512
2. Set the proper permissions on /usr/swap0:
# chmod 0600 /usr/swap0
3. Enable the swap file in /etc/rc.conf:
# swapfile="/usr/swap0" # Set to name of swap file
4. Swap space will be added on system startup. To enable the swap file immediately, specify a free memory device.
# mdconfig -a -t vnode -f /usr/swap0 -u 0 && swapon /dev/md0