resize_device cloud config option can be used to automatically extend the first partition to fill the size of it’s device.
Once the partition has been resized to fill the device, a
/var/lib/rancher/resizefs.done file will be written to prevent the resize tools from being run again. If you need it to run again, delete that file and reboot.
#cloud-config rancher: resize_device: /dev/sda
This behavior is the default when launching RancherOS on AWS.