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
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.
My configuration allows a 2 GB swap file. By default the swap file is
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.