[Bitcoin-development] Chain pruning

Brian Hoffman brianchoffman at gmail.com
Thu Apr 10 17:06:22 UTC 2014

Ok I think I've got a good understanding of where we're at now. I can
promise that the next person to waste your time in 30 days will not be me.
I'm pleasantly surprised to see a community that doesn't kickban newcomers
and takes the time to explain (re-explain) concepts.

Hoping to add *beneficial* thoughts in the future!

On Thu, Apr 10, 2014 at 12:59 PM, Pieter Wuille <pieter.wuille at gmail.com>wrote:

> On Thu, Apr 10, 2014 at 6:47 PM, Brian Hoffman <brianchoffman at gmail.com>
> wrote:
> > Looks like only about ~30% disk space savings so I see your point. Is
> there
> > a critical reason why blocks couldn't be formed into "superblocks" that
> are
> > chained together and nodes could serve a specific superblock, which
> could be
> > pieced together from different nodes to get the full blockchain? This
> would
> > allow participants with limited resources to serve full portions of the
> > blockchain rather than limited pieces of the entire blockchain.
> As this is a suggestion that I think I've seen come up once a month
> for the past 3 years, let's try to answer it thoroughly.
> The actual "state" of the blockchain is the UTXO set (stored in
> chainstate/ by the reference client). It's the set of all unspent
> transaction outputs at the currently active point in the block chain.
> It is all you need for validating future blocks.
> The problem is, you can't just give someone the UTXO set and expect
> them to trust it, as there is no way to prove that it was the result
> of processing the actual blocks.
> As Bitcoin's full node uses a "zero trust" model, where (apart from
> one detail: the order of otherwise valid transactions) it never
> assumes any data received from the outside it valid, it HAS to see the
> previous blocks in order to establish the validity of the current UTXO
> set. This is what initial block syncing does. Nothing but the actual
> blocks can provide this data, and it is why the actual blocks need to
> be available. It does not require everyone to have all blocks, though
> - they just need to have seen them during processing.
> A related, but not identical evolution is merkle UTXO commitments.
> This means that we shape the UTXO set as a merkle tree, compute its
> root after every block, and require that the block commits to this
> root hash (by putting it in the coinbase, for example). This means a
> full node can copy the chain state from someone else, and check that
> its hash matches what the block chain commits to. It's important to
> note that this is a strict reduction in security: we're now trusting
> that the longest chain (with most proof of work) commits to a valid
> UTXO set (at some point in the past).
> In essence, combining both ideas means you get "superblocks" (the UTXO
> set is essentially the summary of the result of all past blocks), in a
> way that is less-than-currently-but-perhaps-still-acceptably-validated.
> --
> Pieter
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