At least for btrfs, very little.

It's also almost instantaneous.

You can think about it like this: the snapshot creates a bunch of hard links to the existing files. So every file basically has 2 pointers to it.

The upgrade now updates the files in the non- snapshot paths, creating new files/keeping the snapshots on the original file. (That's afaik why it's called a CoW filesystem - copy on write)

Now, every changed file will be stored "twice", but all unchanged files are still only on the disk once.

As most upgrades still keep the majority of all files unchanged, there is effectively little space used for the snapshot

definitely going to check that out, thanks