[bitcoin-dev] UTXO growth scaling solution proposal

[Thomas Guyot-Sionnest]

On 21/07/17 03:59 PM, Lucas Clemente Vella via bitcoin-dev wrote:
> 2017-07-21 16:28 GMT-03:00 Major Kusanagi via bitcoin-dev
> <bitcoin-dev at lists.linuxfoundation.org
> <mailto:bitcoin-dev at lists.linuxfoundation.org>>:
> 
>     [...] But the fact is that if we want to make bitcoins last forever,
>     we have the accept unbounded UTXO growth, which is unscalable. So
>     the only solution is to limit UTXO growth, meaning bitcoins cannot
>     last forever. This proposed solution however does not prevent
>     Bitcoin from lasting forever.
> 
>  
> Unless there is a logical contradiction in this phrasing, the proposed
> solution does not improves scalability:
>  - "Bitcoins lasting forever" implies "unscalable";
>  - "not prevent Bitcoin from lasting forever" implies "Bitcoins lasting
> forever";
>  - Thus: "not prevent Bitcoin from lasting forever" implies "unscalable".
> 
> In practice, the only Bitcoin lost would be those whose owners forgot
> about or has lost the keys, because everyone with a significant amount
> of Bitcoins would always shift them around before it loses any luster (I
> wouldn't bother to move my Bitcoins every 10 years). I don't know how to
> estimate the percentage of UTXO is actually lost/forgotten, but I have
> the opinion it isn't worth the hassle.
> 
> As a side note, your estimate talks about block size, which is
> determines blockchain size, which can be "safely" pruned (if you are not
> considering new nodes might want to join the network, in case the full
> history is needed to be stored somewhere). But UTXO size, albeit related
> to the full blockchain size, is the part that currently can not be
> safely pruned, so I don't see the relevance of the analysis.

I think if we wanted to burn lost/stale coins a better approach would be
returning them to miner's as a fee - there will always be lost coins and
miners will be able to get that additional revenue stream as the mining
reward halves. I also don't think we need to worry about doing a gradual
value loss neither, we should just put a limit on UTXO age in block
count (actually I would round it up to 210k blocks as explained below...).


So lets say for example we decide to keep 5 210k blocks "generations"
(that's over 15 years), then on the first block of the 6th generation
all UTXO's from the 1st generation are invalidated and returned into a
"pool".

Given these (values in satoshis):

Pool "P" (invalided UTXO minus total value reclaimed since last halving)
Leftover blocks "B" (210,000 minus blocks mined since last halving)

Then every mined block can reclaim FLOOR(P/B) satoshi in addition to
miner's reward and tx fees.

If the last block of a generation does not get the remainder of the pool
(FLOOR(P/1) == P) it should get carried over.


This would ensure we can clear old blocks after a few generations and
that burnt/lost coins eventually get back in circulation. Also it would
reduce the reliance of miners on actual TX fees.


To avoid excessive miner reward initially, for the first few iterations
the value of B could be increased (I haven't calculated the UTXO size of
the first 210k blocks but it could be excessively high...) or the value
each block can reclaim could be caped (so we would reclaim at an
artificial capacity until the pool depletes...).


Regards,

--
Thomas