Understanding Geth Sync Modes: Fast, Full, and Light Explained
This guide explains Geth's sync mode options—full, fast, and light—including their command‑line flags, how each mode downloads and validates blockchain data, and practical tips such as cache settings for optimal performance.
Sync Mode Classification
--fastEnable fast syncing through state downloads --light Enable light client mode --syncmode full
In Geth 1.6, the --fast flag was renamed to --syncmode=fast, though the old flag still works. The full mode behaves similarly.
Full Sync Steps
Download block headers
Download block bodies
Validate each element starting from the genesis block
Note: this downloads all block data
Fast Sync
Download block headers
Download block bodies
Do not process any transactions until reaching the current block, then take a snapshot and continue like a full node. This sync follows recent transactions and may miss historical data; for example, older incoming transactions to an address may not be retrieved.
When using this mode, set --cache (default 16 MB). It is recommended to increase it to 1 GB (1024)–2 GB (2048).
Light Sync
Only the current state is downloaded. Validation of elements requires requests to a full node.
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