How growing contract storage increases the gas spending?

Question

I have some further questions about this and this threads.

For simplicity, let's imagine that I want to create a cryptocurrency on top of ethereum.

The contract that "handles" this cryptocurrency will have a mapping that will associate addresses to balances.

Now, let's imagine that this turns out as a very popular currency, used by millions of people around the world, making billions of transactions per year.

As more and more people starts "registering" to use this coin, the storage use of the contract grows indefinetly.

Questions:

1. How much of the storage needs to be loaded in memory in order to make the contract execute?
2. Does the size of storage influence the amount of memory needed to execute it?
3. Is all this data always stored in the chain data? Or some of it goes to the state data?
4. How exactly an increasing contract storage also increases gas spending?

Answer 1

Stored data is stored entirely in the state. None of the storage needs to be loaded into VM memory in order to execute transactions on the contract, other than those entries that your contract explicitly loads in code.

As a result, gas costs are independent of the amount of stored data you have (unless you're doing something like iterating over every stored value). You're charged gas for storing or updating a value, charged a much smaller amount of gas to retrieve data from state, and refunded some gas for deleting values from storage.