title: How is state data stored on Ethereum blockchain?
body: <p>Was thinking about storage costs on Ethereum and trying to comprehend the implication of storing a globally standardized credit score (like Bloom) on-chain.</p>
  
  <p>In the simplest case, you would store three 256bit words of data per user, indexed by user addresses, which would mean 4 words of storage.</p>
  
  <p>In the scenario where you reached 100M users, you are looking at 400M words of storage, total. Each word of storage on the Ethereum blockchain requires 20K in gas fees, so with even 1 gwei at the current price of $1147/ETH, you are looking at ~$9.4M to store all credit scores of the user base.</p>
  
  <p>So, my question is thus:</p>
  
  <p>If you change the state of the contract above by updating one user's score, does it require storing the entire 400M words of data again (and incurring the $9.4M charge), or is only a delta of the state stored in some metadata layer?</p>