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> doesn't bitcoin also have a feature similar to this? except bitcoin's pruned node takes ~3GB rather than etherum's 20GB.

No, not in the same way. A pruned Bitcoin node loses validation/state that you can't reconstruct, it does not have the same security properties. See the links in my comment.

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Yes, it is in the same way. In both cases you keep enough state to verify future transactions but not enough to rebuild up to the current state if it were to get corrupted. I don't see anything in your links that says otherwise.

The difference is that it's possible for new Etherium nodes to sync up using just the current state without having to trust the node they got it from or process the history, whereas new Bitcoin nodes have to either process the entire transaction history themselves or trust someone to give them a valid state copy. Also, pruned Bitcoin nodes are more analogous to Etherium nodes which only store the current state without any history (~1GB according to that link). I think the pruned nodes vbuterin talks about are roughly equivalent to unpruned Bitcoin nodes.

Nope, Ethereum fast sync has to trust that peers don't give it an invalid state (it checks that all it's peers have the same state, but that is much weaker protection). https://github.com/ethereum/go-ethereum/pull/1889

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