Ethereum Blockchain Data – What is Stored on the Ethereum Blockchain

Data structures are stored in Merkle Patricia tries (read this and this), usually inside a LevelDB store. The chaindata is exactly that.

I think the structure might slightly depend on the actual node implementation?

Here are two of the trie implementations, could be a good starting point:

• in Go (geth): https://github.com/ethereum/go-ethereum/tree/master/trie
• in Javascript: https://github.com/ethereumjs/merkle-patricia-tree

Once that is understood, you will need to look up the data structures sent over the network, especially blocks, transactions, accounts and storage. Accounts refer to both externally owned and contract accounts, the latter with contract code. Storage refers to contract storage.

Back to the point of data analysis

You might be better off building a traditional NoSQL or SQL database if you want to have quick lookups about any transactions done by an account or contract. What the chaindata is optimised for is validating transactions and maintaining a valid state, and not so for historical lookups.

The answer depends on the client you are using and the mode that you use to sync.

In "full" mode, you do indeed download the entire blockchain, including every transaction, and verify every transaction individually as you go.

In many variants of "fast" mode, only the block headers (metadata), transaction receipts, and state for each block are downloaded. This means you can still see the state at every point in time, but don't need to store the data for every transaction. Additionally, the proof of work is verified for each block to ensure that the blocks are genuine.

In modes like Parity's "warp" sync, you download full "snapshots" of the blockchain which are even smaller than the block header + states. The client then fills in missing data in the background and as needed.

The reason that the full blockchain is so small (only a few GBs) is that putting data on the blockchain is simply very expensive. See What is the cost to store 1KB, 10KB, 100KB worth of data into the ethereum blockchain?