[bitcoin-dev] Small Nodes: A Better Alternative to Pruned Nodes

Aymeric Vitte vitteaymeric at gmail.com
Thu Apr 20 23:42:03 UTC 2017

??? what do you mean? (https://www.soyoustart.com/fr/serveurs-essential/)

Le 20/04/2017 à 17:50, Erik Aronesty via bitcoin-dev a écrit :
> Try to find 1TB dedicated server hosting ...
> If you want to set up an ecommerce site somewhere besides your living
> room, storage costs are still a concern.
> On Mon, Apr 17, 2017 at 3:11 AM, Danny Thorpe via bitcoin-dev
> <bitcoin-dev at lists.linuxfoundation.org
> <mailto:bitcoin-dev at lists.linuxfoundation.org>> wrote:
>     1TB HDD is now available for under $40 USD.  How is the 100GB
>     storage requirement preventing anyone from setting up full nodes?
>     On Apr 16, 2017 11:55 PM, "David Vorick via bitcoin-dev"
>     <bitcoin-dev at lists.linuxfoundation.org
>     <mailto:bitcoin-dev at lists.linuxfoundation.org>> wrote:
>         *Rationale:*
>         A node that stores the full blockchain (I will use the term
>         archival node) requires over 100GB of disk space, which I
>         believe is one of the most significant barriers to more people
>         running full nodes. And I believe the ecosystem would benefit
>         substantially if more users were running full nodes.
>         The best alternative today to storing the full blockchain is
>         to run a pruned node, which keeps only the UTXO set and throws
>         away already verified blocks. The operator of the pruned node
>         is able to enjoy the full security benefits of a full node,
>         but is essentially leeching the network, as they performed a
>         large download likely without contributing anything back.
>         This puts more pressure on the archival nodes, as the archival
>         nodes need to pick up the slack and help new nodes bootstrap
>         to the network. As the pressure on archival nodes grows, fewer
>         people will be able to actually run archival nodes, and the
>         situation will degrade. The situation would likely become
>         problematic quickly if bitcoin-core were to ship with the
>         defaults set to a pruned node.
>         Even further, the people most likely to care about saving
>         100GB of disk space are also the people least likely to care
>         about some extra bandwidth usage. For datacenter nodes, and
>         for nodes doing lots of bandwidth, the bandwidth is usually
>         the biggest cost of running the node. For home users however,
>         as long as they stay under their bandwidth cap, the bandwidth
>         is actually free. Ideally, new nodes would be able to
>         bootstrap from nodes that do not have to pay for their
>         bandwidth, instead of needing to rely on a decreasing
>         percentage of heavy-duty archival nodes.
>         I have (perhaps incorrectly) identified disk space consumption
>         as the most significant factor in your average user choosing
>         to run a pruned node or a lite client instead of a full node.
>         The average user is not typically too worried about bandwidth,
>         and is also not typically too worried about initial blockchain
>         download time. But the 100GB hit to your disk space can be a
>         huge psychological factor, especially if your hard drive only
>         has 500GB available in the first place, and 250+ GB is already
>         consumed by other files you have.
>         I believe that improving the disk usage situation would
>         greatly benefit decentralization, especially if it could be
>         done without putting pressure on archival nodes.
>         *Small Nodes Proposal:*
>         I propose an alternative to the pruned node that does not put
>         undue pressure on archival nodes, and would be acceptable and
>         non-risky to ship as a default in bitcoin-core. For lack of a
>         better name, I'll call this new type of node a 'small node'.
>         The intention is that bitcoin-core would eventually ship
>         'small nodes' by default, such that the expected amount of
>         disk consumption drops from today's 100+ GB to less than 30 GB.
>         My alternative proposal has the following properties:
>         + Full nodes only need to store ~20% of the blockchain
>         + With very high probability, a new node will be able to
>         recover the entire blockchain by connecting to 6 random small
>         node peers.
>         + An attacker that can eliminate a chosen+ 95% of the full
>         nodes running today will be unable to prevent new nodes from
>         downloading the full blockchain, even if the attacker is also
>         able to eliminate all archival nodes. (assuming all nodes
>         today were small nodes instead of archival nodes)
>         Method:
>         A small node will pick an index [5, 256). This index is that
>         node's permanent index. When storing a block, instead of
>         storing the full block, the node will use Reed-Solomon coding
>         to erasure code the block using a 5-of-256 scheme. The result
>         will be 256 pieces that are 20% of the size of the block each.
>         The node picks the piece that corresponds to its index, and
>         stores that instead. (Indexes 0-4 are reserved for archival
>         nodes - explained later)
>         The node is now storing a fragment of every block. Alone, this
>         fragment cannot be used to recover any piece of the
>         blockchain. However, when paired with any 5 unique fragments
>         (fragments of the same index will not be unique), the full
>         block can be recovered.
>         Nodes can optionally store more than 1 fragment each. At 5
>         fragments, the node becomes a full archival node, and the
>         chosen indexes should be 0-4. This is advantageous for the
>         archival node as the encoded data for the first 5 indexes will
>         actually be identical to the block itself - there is no
>         computational overhead for selecting the first indexes. There
>         is also no need to choose random indexes, because the full
>         block can be recovered no matter which indexes are chosen.
>         When connecting to new peers, the indexes of each peer needs
>         to be known. Once peers totaling 5 unique indexes are
>         discovered, blockchain download can begin. Connecting to just
>         5 small node peers provides a >95% chance of getting 5
>         uniques, with exponentially improving odds of success as you
>         connect to more peers. Connecting to a single archive node
>         guarantees that any gaps can be filled.
>         A good encoder should be able to turn a block into a 5-of-256
>         piece set in under 10 milliseconds using a single core on a
>         standard consumer desktop. This should not slow down initial
>         blockchain download substantially, though the overhead is more
>         than a rounding error.
>         *DoS Prevention:*
>         A malicious node may provide garbage data instead of the
>         actual piece. Given just the garbage data and 4 other correct
>         pieces, it is impossible (best I know anyway) to tell which
>         piece is the garbage piece.
>         One option in this case would be to seek out an archival node
>         that could verify the correctness of the pieces, and identify
>         the malicious node.
>         Another option would be to have the small nodes store a
>         cryptographic checksum of each piece. Obtaining the
>         cryptographic checksum for all 256 pieces would incur a
>         nontrivial amount of hashing (post segwit, as much as 100MB of
>         extra hashing per block), and would require an additional ~4kb
>         of storage per block. The hashing overhead here may be
>         prohibitive.
>         Another solution would be to find additional pieces and
>         brute-force combinations of 5 until a working combination was
>         discovered. Though this sounds nasty, it should take less than
>         five seconds of computation to find the working combination
>         given 5 correct pieces and 2 incorrect pieces. This
>         computation only needs to be performed once to identify the
>         malicious peers.
>         I also believe that alternative erasure coding schemes exist
>         which actually are able to identify the bad pieces given
>         sufficient good pieces, however I don't know if they have the
>         same computational performance as the best Reed-Solomon coding
>         implementations.
>         *Deployment:*
>         Small nodes are completely useless unless the critical mass of
>         5 pieces can be obtained. The first version that supports
>         small node block downloads should default everyone to an
>         archival node (meaning indexes 0-4 are used)
>         Once there are enough small-node-enabled archive nodes, the
>         default can be switched so that nodes only have a single index
>         by default. In the first few days, when there are only a few
>         small nodes, the previously-deployed archival nodes can help
>         fill in the gaps, and the small nodes can be useful for
>         blockchain download right away.
>         ----------------------------------
>         This represents a non-trivial amount of code, but I believe
>         that the result would be a non-trivial increase in the
>         percentage of users running full nodes, and a healthier
>         overall network.
>         _______________________________________________
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