There are three main synchronization modes

• fast sync

  • downloads only the latest state, headers, and optionally bodies and receipts

• beam sync

  • same as fast sync but also allows to query the chain within the first few minutes from starting

• archive sync

  • heavy historical sync verifying all the transactions and keeping all the historical state

Fast Sync

Fast sync is probably the most popular method of syncing. After completing the fast sync your node will have the ability to answer questions like 'what is my account balance now', 'how many XYZ tokens SomeExchange holds at the moment'.

Fast sync is the default syncing method in Nethermind so whenever you launch standard configs you will end up using Fast Sync.

./Nethermind.Runner --config mainnet

./Nethermind.Runner --config goerli

Fast sync has multiple stages. Nethermind uses a pivot block number to improve fast sync performance. The pivot block data is provided in the configuration file and consists of the block number, block hash and block total difficulty (please note that total difficulty is different than difficulty). Please note that the meaning of pivot block is different in Nethermind than in other clients, for example. Before synchronizing state data Nethermind synchronizes in two directions - backwards from pivot block to 0 for headers and forward to the head of the chain for headers, blocks and receipts. Forward sync might be very slow (5 - 50 bps) so it is important that you use the latest config or update your config before synchronization.

After downloading the block data Nethermind will start state sync (downloading the latest state trie nodes). We are providing an estimate for the download size and progress but the real value may be different than the estimate (especially if you are using an old version of Nethermind).

After the state sync finishes you will see the 'Processed...' messages like in archive sync - it means that your node is in sync and is processing the latest blocks.

Mainnet sync, at the time of writing (July 2020), takes around 6 hours on an UpCloud $40 VM (and then syncs receipts and bodies in the background if you enabled them in the configuration. Goerli sync should take around 40 minutes.

State sync log messages have multiple values displayed. First dd.HH:mm:ss total state sync time is displayed, followed by an estimated sync progress (percentage of total database data synced), then the current download speed is displayed (there will be times when it will slow down to 0 or single digit numbers, especially towards the end of the sync). In general 6 hours sync times that we achieve are with around 2000 kB/s (kilobytes per second). You can calculate it on the example as ~45GB / (2MB/s) ~ 22500 seconds ~6.25 hours. We also display the number of state accounts synced, number of trie nodes synced and a diagnostic message in the format of [number_of_pending_requests].[average time spent in response handler displayed as milliseconds]. So 5.6.20ms means that we are awaiting for responses to 5 requests that have been sent to peers and the average time it takes us to process a single response is 6.20ms. The response handling times will differ depending on how many trie nodes are already cached (so they will be significantly slower for a while after the node restart when cache has to be rebuilt) and based on how fast the database IO is (SSD vs NVMe vs cloud drives). For a reasonable sync time you probably should expect these values to be below 15ms (but they may be as high as 700ms for a while after restarting the node).

Beam Sync

The simplest way of explaining beam sync is by saying that the beam sync is exactly the same as fast sync but additionally it downloads the state data (witnesses) for the latest blocks. It also allows to execute some queries about the current state via JSON RPC much before the actual fast sync finishes. Currently beam sync takes more resources than fast sync and slows the fast sync down significantly but it allows you the query blockchain within a few minutes from starting.

When the 'Beam sync is ON' message is displayed then it generally means that you can ask about the latest headers, latest transactions, you can ask about eth_getBalance, eth_getCode, eth_call, debug_trace, trace_replayTransactionsetc. You cannot ask about transaction receipts or logs as most of them are not processed yet. With beam sync it is possible to create and broadcast an Ether or token transfer transactions from the node.

Archive Sync

Archive sync is the 'heaviest' and slowest sync mode but allows to ask questions like 'what was the balance of my account 2 years ago?', 'how many XYZ token were held in SomeExchange custody in 2017?'.

We have prepared default archive sync configurations and they can be launched from Nethermind Launcher (just choose the archive options) or by simply loading appropriate config when launching ./Nethermind.Runner --config mainnet_archive

./Nethermind.Runner --config goerli_archive

While for some smaller networks archive sync can complete very quickly (in minutes or hours) mainnet sync would take 2 - 6 weeks depending on the speed of your IO (whether you use SSD or NVMe or depending on the cloud provider IOPS). Database size in archive sync is the biggest from all modes as you will store all the historical data.

Explanation of some data in the logs:

• at the beginning you may see a 'Waiting for peers...' message while the node is trying to discover nodes that it can sync with.

• 'Downloaded 1234/8000000' shows the number of unprocessed blocks (with transactions) downloaded from the network. For mainnetthis value may be slower than processing at first but very quickly you will see blocks being downloaded much faster than processed. Empty blocks can be as small as 512 bytes (just headers without transactions) and full blocks with heavy transactions can reach a few hundred kilobytes. We display both current download speed (calculated in the last second) and average (total) speed since starting the node.

• 'Processed ...' displays the blocks that have been processed by the EVM. The first number shows the current head block number, then you can see mgasps (million gas per second) - current and total, then tps (transactions per second) - current and total, bps (blocks per second). Then recv queue (transactions signature public key recovery queue), proc queue (processor queue). Both recovery queue and processor queue are designed so when too many blocks are waiting for processing then only their hashes are kept in memory and remaining data are stored in the database. Thus, the queues numbers that you can see will be capped by some number.

• 'Cache for epoch...' informs about ethash cache needed for block seal verification (only on mainnet and ropsten). Caches will be calculated every 30000 blocks (length of an epoch) but can also be calculated for the latest blocks that are being broadcast on the network.

• After the archive sync finishes you will see the 'Processed...' message appearing on average every 15 seconds when the new block is processed.

• mgasps, tps, bps values should not be treated as comparable as they may differ massively on different parts of the chain. For example when blocks are empty you may see very high bps values with very low (or even zero) tps and mgasps values as there are no transactions and no gas for EVM processing and blocks are very light. On the other hand when blocks are filled with very heavy transactions then bps might be very low while mgasps will be very high. It is even possible that you will see a lot of very light transactions where tps will be high while bps and mgasps will be average.