When I worked at Airbus we designed an entire aircraft including all those cad drawings on less than 3TB.
Congratulations.
What the hell are you planning to put on 20TB of storage?
Initially, the 1.2 TB I'm using right now on my 2 TB hard drive. When I got this computer less than years ago (as I'm sure you'll recall vividly, having tried to push me towards buying various things I didn't want), I only had 200 GB. That's 1 TB of growth in under two years, on top of which I want to get back into recording music again which takes up its fair share of space.
On top of that, Ceph provides an Amazon S3-like interface for general-purpose object storage, which I'm considering opening up to the Internet to share files with people. This will be yet another data source on top of what I'm already using (which is growing at more than 1 TB every two years).
A single 4 TB hard drive is only going to last me another couple of years at this rate. A properly built Ceph cluster will last me much longer than that, as well as providing seamless upgradability thanks to Ceph's distributed and highly available nature.
If it makes you feel any better, I'm going to be building the first incarnation of this cluster on 10x 1 TB drives I'm getting for free. That's only 5 TB of storage until I start buying bigger drives.