We were discussing separate partitions for / and /home in an installation and I noticed that my two separate partitions shared the same space. btrfs to the rescue!

https://fedoramagazine.org/working-with-btrfs-subvolumes/

So where do the home and root subvolumes in Fedora Linux come from?

These are created by the installer at install time. Traditional installations would often include a separate filesystem partition for the / and /home directories. During boot, these are then appropriately mounted to assemble one full filesystem. But there is an issue with this approach: Unless you use technologies such as lvm, it is very hard to change a partitions size at some point in the future. As a consequence you may end up in a situation where either your / or /home runs out of space, while the respective other partition has lots of unused, free space left.

Since Btrfs subvolumes are all part of the same filesystem, they will share the space that the underlying filesystem offers. Remember when we created the subvolumes above? We never told Btrfs how big they are: A subvolume can take up all the space the filesystem has, by default nothing keeps it from doing so. However, we could dynamically impose size limits via Btrfs qgroups, which can also be modified during runtime (And we’ll see how in a later article in this series).