Online Resize Ubuntu/Debian Disk

Start by installing cloud-utils. This contains the growpart tool.

sudo apt-get install -y cloud-utils    

The root partition is 32 GB but the device itself is 64GB. I intent to resize the root partition (/dev/sda2)

Run growpart

sudo growpart /dev/sda 2

The first parameter is the device you want to resize, and the second is the partition. Below is the output:

devin@postgres-replica:~$ sudo growpart /dev/sda 2
CHANGED: partition=2 start=4096 old: size=67102720 end=67106816 new: size=134213599 end=134217695

And then:

sudo resize2fs /dev/sda2

Success