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Date: Mon, 3 Aug 2015 08:35:45 +1000
From: Anthony Towns <aj@erisian.com.au>
To: Gavin Andresen <gavinandresen@gmail.com>,
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Subject: Re: [bitcoin-dev] Block size following technological growth
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On Thu, Jul 30, 2015 at 12:20:30PM -0400, Gavin Andresen wrote:
> On Thu, Jul 30, 2015 at 10:25 AM, Pieter Wuille via bitcoin-dev
> > Some things are not included yet, such as a testnet whose size runs ahead
> > of the main chain, and the inclusion of Gavin's more accurate sigop
> > checking after the hard fork.
> First, THANK YOU for making a concrete proposal!
> So we'd get to 2MB blocks in the year 2021. I think that is much too
> conservative, and the most likely effect of being that conservative is that
> the main blockchain becomes a settlement network, affordable only for
> large-value transactions.
I haven't seen anyone do the (trivial) maths on this. Have I just
missed it? By my count:
- blocks in 25 btc in rewards and about 0.5 btc in fees per block
- at ~$300 USD per btc, that's ~$7,650 per block
- current hashrate is ~400 PH/s; so ~240,000 PH/block works out
to having to spend about 30PH per dollar earnt.
- for comparison,
https://products.butterflylabs.com/cloud-mining-contracts.html
quotes $1.99 per GH/s for 12 months, which by my count is
60*60*24*365 / 1.99 GH/$ = 15.8 PH per dollar spent
- hashrate growth has slowed from about x4/quarter to x2/year:
sep '13: ~1PH/s
dec '13: ~4PH/s
mar '14: ~20PH/s
jun '14: ~80PH/s
sep '14: ~200PH/s
aug '15: ~400PH/s
- so, as far as I understand it, miners don't make absurd profits compared
to capital investment and running costs
- presumably, then, miners will stop mining bitcoin if the revenue/block
drops significantly at some point
- less miners means a lower hashrate; a lower hashrate makes
50% attacks easier, and that's a bad thing (especially if there's lots
of pre-loved ASIC mining hardware available cheap on ebay or alibaba)
- in about a year, the block reward halves, cutting out 12.5 btc or
~$3750 USD per block. without an increase in fees per block, miners
will just get ~$3900 USD per block
- the last time the reward for mining a block was under $4000 per block
was around oct '13, with a hashrate of ~2PH/s
- 13 btc in fees per block compared to .5 btc in fees per block is a
25x increase; which could be either an increase in fee/txn or
txns/block
- with ~500 bytes/transaction, that's ~2000 transactions per MB
- 13 btc in fees ($3900) per block means per transaction fees of
about
$2 for 1MB blocks
$1 for 2MB blocks
25c for 8MB blocks
10c for 20MB blocks
(assuming full blocks, 500 byte txns)
- comparing that to credit card or paypal fees at ~2.5% that's:
$2 -> minimum transaction $80
$1 -> minimum transaction $40
25c -> minimum transaction $10
10c -> minimum transaction $4
- those numbers only depend on the USD/BTC exchange rate in so far as
the more USD for a BTC, the more likely the block reward will pay
for hashrate without transaction fees, even with the reward reduced
to 12.5 btc/block. otherwise it's just USD/txn paying for USD/hashrate
- the reference implementation fee of 0.1mBTC/kB equates to about
3c per transaction (since it rounds up). Even 10c/transaction is more
than a 3x increase on that.
What the above says to me is that even assuming everyone starts paying
fees, the lightning network works great, and so do sidechains and whatever
else, you /still/ want to up the volume of bitcoin txns by something like
an order of magnitude above what's currently allowed within a year or so.
Cheers,
aj
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