[Ethereum] Geth’s “fast” sync, and why is it faster


One of the answers to this question suggested using Geth's --fast flag to help quickly synchronise the block data.

How does the flag work, and how does using it speed up the synchronisation? Are we syncing less data, or are we in some way performing fewer checks on its integrity or source?


As of Geth version 1.6.0, the --fast flag has become --syncmode=fast (though --fast is also still usable for now).

Best Answer

There's a lot of detail on this PR on github. Here's a quote:

Instead of processing the entire block-chain one link at a time, and replay all transactions that ever happened in history, fast syncing downloads the transaction receipts along the blocks, and pulls an entire recent state database.