Space reservation system

Question

I am writing an webapp where users can sign up to. Every user hass 2GB of disk space for free use. Now ofc. there will be users that don't use their space at all or barely and it would be wasterfull to reserve 2 full gb. How could every user have thoose 2 gb without wasting disk space

Answer 1

Maybe you create a little packet whenever a user Uploads something; this is lied at any place on the disk, and the place is saved. When then sb wants to Upload, you check if the sum of all packets is smaller than 2GB.
This is just an idea, I'm not familiar with this.