I know we cannot use Smart contract to be used as large database. But my client is asking for proof. Can you guys provide any link that will support this.
What you know might be wrong. There is nothing stopping you from using smart contracts as a large database.
But, if you want to store large amounts of data on blockchain, it will require a large amount of gas. And for each gas, if your are providing
20 gwei as gas price, then you may be required to pay large amount of ether as fees of transactions.
However, this too can be avoided if there are miners which are mining transactions which have significantly low gas price or even zero gas price. In such case, you may use smart contracts as a large database for free. But you may not always rely on getting your
0 gas price transaction mined.
Zero gas price transactions get sometimes mined, sometimes not. There is zero incentive for miners to include them so they obviously are not prioritized very high.
So, in reality, if you want to get your transaction through you have to pay. You can find some calculations easily online; here's some discussion: How can I estimate price of data storage? and https://www.reddit.com/r/ethereum/comments/6rj8ks/current_storage_costs_and_limits/