This post is older than a year. Consider some information might not be accurate anymore.
When you migrate your root to an external usb hdd, the next step is to enable swap.
Let’s see the used and free memory with free -h
root@pelion:/boot# free -h
total used free shared buffers cached
Mem: 973M 858M 115M 0B 28M 703M
Swap: 0B 0B 0B
As you can see there is no swap available. Since the previous root was on the SD card, it is a bad idea for swapping, as result the micro SD card will wear out. Using an external usb hdd allows us to use swap on the hdd, if the Raspberry Pi needs to swap. Raspian uses a script dphys-swapfile to manage swap. The configuration file is located at /etc/dphys-swapfile. Raspian doesn’t need a separate swap partition.
CONF_SWAPSIZE=2048
My configuration allows a 2 GB swap file. By default the swap file is /var/swap.
Show active swap with summary option
root@pelion:~# swapon -s
Filename Type Size Used Priority
/var/swap file 2097148 0 -1
To deactivate swap and activate swap again
swapoff /var/swap
swapon /var/swap
Usually the Raspberry Pi won’t need swapping, but in case it needs to, it is capable now.
