Sync modes

There are three main synchronization modes
snap sync
fastest way to sync to a network (Syncs to Mainnet in ~3 hours)
fast sync
downloads only the latest state, headers, and optionally bodies and receipts
you can run it like this: ./Nethermind.Runner --config mainnet
if you have a good connection and a reasonable machine then setting --Network.MaxActivePeers to 256 (see NetworkConfig secion in configuration) should give much better fast sync times (we use 256 peers to get 5 hours syncs)
https://docs.nethermind.io/nethermind/ethereum-client/configuration/network​
archive sync
heavy historical sync verifying all the transactions and keeping all the historical state
you can run it like this ./Nethermind.Runner --config mainnet_archive
Sync Mode
Disk Space needed
Full current state
Full current and all historical states
Can sync a full archive node from this
Time to sync
Time to RPC
archive
~12TB
YES
YES
YES
~3 weeks
~3 weeks
fast sync with all bodies and receipts
~500GB
YES
NO
YES
~20 hours
~20 hours
default fast sync (with barriers set to support Eth2)
~100GB
YES
NO
NO
~11 hours
~11 hours
fast sync without receipts
~130GB
YES
NO
YES
~12 hours
~12 hours
fast sync without bodies and receipts
~70GB
YES
NO
NO
~9 hours
~9 hours
Snap Sync
The Nethermind client includes beta Snap Sync support from version v1.13.0. This allows a node to perform the initial synchronization and download of Ethereum’s state up to 10 times faster than before.
How to Enable
To enable make sure SnapSync is set to true in the Sync module of your .cfg file
"Sync": {
	"SnapSync": true
}
IMPORTANT: Do not enable snap sync on a previously synced node. Only use when syncing to the network for the first time.
IMPORTANT: Do not remove other configurations from Sync module. Just add "SnapSync": true, e.g.
"Sync": {
	"SnapSync": true,
	"PivotNumber": 15011000,
        "PivotHash": "0x46c838d02d5fa5bc070080ed7965da1d888f6eb1797045365407c7011280af56",
    	"PivotTotalDifficulty": "52367203434576253689712",
    	"FastBlocks": true
}
​
Snap Sync VS Other Sync Modes
More than 10TB of storage is needed today to run a full archive node — one that stores all the state since genesis. This makes it so that setting up a node can take days if not weeks.
An operator can instead choose to run Fast Sync when setting up the node, which downloads the state associated with the last 64 blocks that currently comes at about 90Gb. This can still take more than 24 hours in a fast machine. With Snap Sync the sync time is reduced to 2-3h with a download of 30Gb.
This reduction in sync time and download size has to do with the specific way in which Ethereum’s state is stored in a node: Merkle trees.
With Fast Sync, a node downloads the headers of each block and retrieves all the nodes beneath it until it reaches the leaves. By contrast, Snap Sync only downloads the leaf nodes, generating the remaining nodes locally which saves time and packets downloaded.
Current limitations and future development 
As of v1.13.0, Snap Sync on the Nethermind client can only download the Ethereum state but not serve it to other clients implementing Snap Sync.
Since the only Ethereum client that supports serving Snap Sync requests is Geth, only networks supported by Geth can be synced: Mainnet, Goerli, Ropsten and Rinkeby.
Also, as stated above, it is not advised to enable Snap Sync on already synced node as it can trigger syncing again. This will be fixed in the next update.
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 as we sometimes manually adjust the estimator based on our observations of the chain growth rate). Because of this sometimes your sync may continue even when it shows ~100% finished. The other important component is the speed of your sync - if your IO / network / file system causes the state sync to go much slower than around 1.5MB per second on average then you will start downloading some parts of the trie over and over again. In such cases you may be surprised to see something like 58000MB / 53000MB (100%). It means that you downloaded around 5GB of data that is no longer needed. If your sync is very slow (extended beyond two days) then very likely your setup cannot catch up with the chain progress.
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 (December 2020), takes around 8 hours on an UpCloud 16GB RAM 8 CPU  $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.
Fast sync logs example for mainnet.
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 shown on screenshots are with around 2000 kB/s (kilobytes per second) average sync rate. You can calculate it in 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).
A single restart of the node during the fast sync may extend the sync time by up to two hours because the node has to rebuild the caches by reading millions of values from the database.
At the last stages of the sync the node will be repeatedly displaying the branch sync progress and changing the block number to which it tries to catch up. This stage should take between 30 minutes and two hours. If it lasts much more then it is possible that you will not be able to catch up with the network progress.
One of the best indicators that you are close to be synced is combined ~100% state size progress and nearly 100% branch sync progress.
​
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.
Example of the archive sync logs
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.
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Outline
There are three main synchronization modes
Snap Sync
Fast Sync
Archive Sync