The initial block download

After this lesson you can: watch your own machine rebuild the state of Bitcoin from the genesis block and confirm the supply it computed.

What is happening

Your node first downloads all block headers from peers, which gives it the chain's shape and total work. Then it fetches blocks in parallel from several peers and verifies each one in order: proof of work, every transaction, every signature (for recent blocks), building the UTXO set as it goes. At the end it holds the complete current state of Bitcoin, computed entirely by itself.

The initial block download: headers first, then blocks in parallel, verified in order into the UTXO set. — How the initial sync works.

Steps

01Start the node and let it find peers. The dashboard or bitcoin-cli getblockchaininfo shows progress as a fraction of the chain verified.
02Leave it running. Early blocks fly by; blocks from 2017 onward are full and verification slows. Expect several hours on a fast desktop, one to three days on a Raspberry Pi with an SSD.
03If the node uses assumevalid (the default), signatures in blocks before a hard-coded recent block are not re-verified, which saves most of the time; everything else is still checked. You can disable it for a full re-verification.
04Newer Core versions offer AssumeUTXO: load a recent UTXO snapshot, start using the node at once, and verify the full history in the background.
05When progress reaches 100% and the tip matches a public explorer's height, you are synced. Your node will now stay within a block of the network.

Check your work

Run getblockchaininfo (or look at the dashboard). The block count should match the network height and verificationprogress should be essentially 1. Run gettxoutsetinfo to have your node report the total supply it computed. That number is 21 million being verified by your own machine.

Two shortcuts compared: assumevalid skips old signature checks; AssumeUTXO starts from a snapshot and verifies history in the background. — Two shortcuts, and what each still checks.

MINI CHALLENGEVerify the supply

At block 840,000 (the April 2024 halving), roughly how many bitcoin had been issued, in millions? Era totals: 10.5 + 5.25 + 2.625 + 1.3125.

million BTC

CHECK YOURSELF

1/3 What does the node hold when the initial sync completes?The complete verified current state: every unspent outputA summary of the chain downloaded from a trusted serverOnly the latest block and its transactions

2/3 What does assumevalid skip?All verification of blocks older than a year, which are downloaded from a trusted serverSignature checks for blocks before a known recent block; structure and chain are still verifiedProof-of-work checks for the early blocks, which are trusted because they are old

3/3 What does your node download first during the initial sync?All block headers, which give it the chain's shape and total workOnly the most recent thousand blocksThe complete UTXO set from a trusted server

pick an answer for each question

The initial block download

What is happening

Steps

Check your work

SOURCES