How to online grow a ext4 partition

This tutorial uses Arch Linux so package names might vary

Install cloud-guest-utils which includes growpart Backup your partition table so you can recover if needed
sudo sfdisk -d /dev/sda > partition_bak.dmp Use growpart to grow the partition
growpart /dev/sda 2 Use resize2fs to grow your filesystem
resize2fs /dev/sda2

Now you should have a ext4 partition that spans the entire rest of your drive.