The Origins of the (Modern) Blocksize Debate

On May 4, 2015, Gavin Andresen wrote on his blog:

I was planning to submit a pull request to the 0.11 release of Bitcoin Core that will allow miners to create blocks bigger than one megabyte, starting a little less than a year from now. But this process of peer review turned up a technical issue that needs to get addressed, and I don’t think it can be fixed in time for the first 0.11 release.
I will be writing a series of blog posts, each addressing one argument against raising the maximum block size, or against scheduling a raise right now… please send me an email (g… if I am missing any arguments

In other words, Gavin proposed a hard fork via a series of blog posts, bypassing all developer communication channels altogether and asking for personal, private emails from anyone interested in discussing the proposal further.

On May 5 (1 day after Gavin submitted his first blog post), Mike Hearn published The capacity cliff on his Medium page. 2 days later, he posted Crash landing. In these posts, he argued:

A common argument for letting Bitcoin blocks fill up is that the outcome won’t be so bad: just a market for fees… this is wrong. I don’t believe fees will become high and stable if Bitcoin runs out of capacity. Instead, I believe Bitcoin will crash.
…a permanent backlog would start to build up… as the backlog grows, nodes will start running out of memory and dying… as Core will accept any transaction that’s valid without any limit a node crash is eventually inevitable.

He also, in the latter article, explained that he disagreed with Satoshi’s vision for how Bitcoin would mature[1][2]:

Neither me nor Gavin believe a fee market will work as a substitute for the inflation subsidy.

Gavin continued to publish the series of blog posts he had announced while Hearn made these predictions. [1][2][3][4][5][6][7]

Matt Corallo brought Gavin’s proposal up on the bitcoin-dev mailing list after a few days. He wrote:

Recently there has been a flurry of posts by Gavin at which advocate strongly for increasing the maximum block size. However, there hasnt been any discussion on this mailing list in several years as far as I can tell…
So, at the risk of starting a flamewar, I’ll provide a little bait to get some responses and hope the discussion opens up into an honest comparison of the tradeoffs here. Certainly a consensus in this kind of technical community should be a basic requirement for any serious commitment to blocksize increase.
Personally, I’m rather strongly against any commitment to a block size increase in the near future. Long-term incentive compatibility requires that there be some fee pressure, and that blocks be relatively consistently full or very nearly full. What we see today are transactions enjoying next-block confirmations with nearly zero pressure to include any fee at all (though many do because it makes wallet code simpler).
This allows the well-funded Bitcoin ecosystem to continue building systems which rely on transactions moving quickly into blocks while pretending these systems scale. Thus, instead of working on technologies which bring Bitcoin’s trustlessness to systems which scale beyond a blockchain’s necessarily slow and (compared to updating numbers in a database) expensive settlement, the ecosystem as a whole continues to focus on building centralized platforms and advocate for changes to Bitcoin which allow them to maintain the status quo

Shortly thereafter, Corallo explained further:

The point of the hard block size limit is exactly because giving miners free rule to do anything they like with their blocks would allow them to do any number of crazy attacks. The incentives for miners to pick block sizes are no where near compatible with what allows the network to continue to run in a decentralized manner.

Tier Nolan considered possible extensions and modifications that might improve Gavin’s proposal and argued that soft caps could be used to mitigate against the dangers of a blocksize increase. Tom Harding voiced support for Gavin’s proposal

Peter Todd mentioned that a limited blocksize provides the benefit of protecting against the “perverse incentives” behind potential block withholding attacks.

Slush didn’t have a strong opinion one way or the other, and neither did Eric Lombrozo, though Eric was interested in developing hard-fork best practices and wanted to:

explore all the complexities involved with deployment of hard forks. Let’s not just do a one-off ad-hoc thing.

Matt Whitlock voiced his opinion:

I’m not so much opposed to a block size increase as I am opposed to a hard fork… I strongly fear that the hard fork itself will become an excuse to change other aspects of the system in ways that will have unintended and possibly disastrous consequences.

Bryan Bishop strongly opposed Gavin’s proposal, and offered a philosophical perspective on the matter:

there has been significant public discussion… about why increasing the max block size is kicking the can down the road while possibly compromising blockchain security. There were many excellent objections that were raised that, sadly, I see are not referenced at all in the recent media blitz. Frankly I can’t help but feel that if contributions, like those from #bitcoin-wizards, have been ignored in lieu of technical analysis, and the absence of discussion on this mailing list, that I feel perhaps there are other subtle and extremely important technical details that are completely absent from this — and other — proposals.
Secured decentralization is the most important and most interesting property of bitcoin. Everything else is rather trivial and could be achieved millions of times more efficiently with conventional technology. Our technical work should be informed by the technical nature of the system we have constructed.
There’s no doubt in my mind that bitcoin will always see the most extreme campaigns and the most extreme misunderstandings… for development purposes we must hold ourselves to extremely high standards before proposing changes, especially to the public, that have the potential to be unsafe and economically unsafe.
There are many potential technical solutions for aggregating millions (trillions?) of transactions into tiny bundles. As a small proof-of-concept, imagine two parties sending transactions back and forth 100 million times. Instead of recording every transaction, you could record the start state and the end state, and end up with two transactions or less. That’s a 100 million fold, without modifying max block size and without potentially compromising secured decentralization.
The MIT group should listen up and get to work figuring out how to measure decentralization and its security.. Getting this measurement right would be really beneficial because we would have a more academic and technical understanding to work with.

Gregory Maxwell echoed and extended that perspective:

When Bitcoin is changed fundamentally, via a hard fork, to have different properties, the change can create winners or losers…
There are non-trivial number of people who hold extremes on any of these general belief patterns; Even among the core developers there is not a consensus on Bitcoin’s optimal role in society and the commercial marketplace.
there is a at least a two fold concern on this particular (“Long term Mining incentives”) front:
One is that the long-held argument is that security of the Bitcoin system in the long term depends on fee income funding autonomous, anonymous, decentralized miners profitably applying enough hash-power to make reorganizations infeasible.
For fees to achieve this purpose, there seemingly must be an effective scarcity of capacity.
The second is that when subsidy has fallen well below fees, the incentive to move the blockchain forward goes away. An optimal rational miner would be best off forking off the current best block in order to capture its fees, rather than moving the blockchain forward…
tools like the Lightning network proposal could well allow us to hit a greater spectrum of demands at once — including secure zero-confirmation (something that larger blocksizes reduce if anything), which is important for many applications. With the right technology I believe we can have our cake and eat it too, but there needs to be a reason to build it; the security and decentralization level of Bitcoin imposes a hard upper limit on anything that can be based on it.
Another key point here is that the small bumps in blocksize which wouldn’t clearly knock the system into a largely centralized mode — small constants — are small enough that they don’t quantitatively change the operation of the system; they don’t open up new applications that aren’t possible today
the procedure I’d prefer would be something like this: if there is a standing backlog, we-the-community of users look to indicators to gauge if the network is losing decentralization and then double the hard limit with proper controls to allow smooth adjustment without fees going to zero (see the past proposals for automatic block size controls that let miners increase up to a hard maximum over the median if they mine at quadratically harder difficulty), and we don’t increase if it appears it would be at a substantial increase in centralization risk. Hardfork changes should only be made if they’re almost completely uncontroversial — where virtually everyone can look at the available data and say “yea, that isn’t undermining my property rights or future use of Bitcoin; it’s no big deal”. Unfortunately, every indicator I can think of except fee totals has been going in the wrong direction almost monotonically along with the blockchain size increase since 2012 when we started hitting full blocks and responded by increasing the default soft target. This is frustrating
many people — myself included — have been working feverishly hard behind the scenes on Bitcoin Core to increase the scalability. This work isn’t small-potatoes boring software engineering stuff; I mean even my personal contributions include things like inventing a wholly new generic algebraic optimization applicable to all EC signature schemes that increases performance by 4%, and that is before getting into the R&D stuff that hasn’t really borne fruit yet, like fraud proofs. Today Bitcoin Core is easily >100 times faster to synchronize and relay than when I first got involved on the same hardware, but these improvements have been swallowed by the growth. The ironic thing is that our frantic efforts to keep ahead and not lose decentralization have both not been enough (by the best measures, full node usage is the lowest its been since 2011 even though the user base is huge now) and yet also so much that people could seriously talk about increasing the block size to something gigantic like 20MB. This sounds less reasonable when you realize that even at 1MB we’d likely have a smoking hole in the ground if not for existing enormous efforts to make scaling not come at a loss of decentralization.

Peter Todd also summarized some academic findings on the subject:

In short, without either a fixed blocksize or fixed fee per transaction Bitcoin will will not survive as there is no viable way to pay for PoW security. The latter option — fixed fee per transaction — is non-trivial to implement in a way that’s actually meaningful — it’s easy to give miners “kickbacks” — leaving us with a fixed blocksize.
Even a relatively small increase to 20MB will greatly reduce the number of people who can participate fully in Bitcoin, creating an environment where the next increase requires the consent of an even smaller portion of the Bitcoin ecosystem. Where does that stop? What’s the proposed mechanism that’ll create an incentive and social consensus to not just ‘kick the can down the road’(3) and further centralize but actually scale up Bitcoin the hard way?

Some developers (e.g. Aaron Voisine) voiced support for Gavin’s proposal which repeated Mike Hearn’s “crash landing” arguments.

Pieter Wuille said:

I am — in general — in favor of increasing the size blocks…
Controversial hard forks. I hope the mailing list here today already proves it is a controversial issue. Independent of personal opinions pro or against, I don’t think we can do a hard fork that is controversial in nature. Either the result is effectively a fork, and pre-existing coins can be spent once on both sides (effectively failing Bitcoin’s primary purpose), or the result is one side forced to upgrade to something they dislike — effectively giving a power to developers they should never have. Quoting someone: “I did not sign up to be part of a central banker’s committee”.
The reason for increasing is “need”. If “we need more space in blocks” is the reason to do an upgrade, it won’t stop after 20 MB. There is nothing fundamental possible with 20 MB blocks that isn’t with 1 MB blocks.
Misrepresentation of the trade-offs. You can argue all you want that none of the effects of larger blocks are particularly damaging, so everything is fine. They will damage something (see below for details), and we should analyze these effects, and be honest about them, and present them as a trade-off made we choose to make to scale the system better. If you just ask people if they want more transactions, of course you’ll hear yes. If you ask people if they want to pay less taxes, I’m sure the vast majority will agree as well.
Miner centralization. There is currently, as far as I know, no technology that can relay and validate 20 MB blocks across the planet, in a manner fast enough to avoid very significant costs to mining. There is work in progress on this (including Gavin’s IBLT-based relay, or Greg’s block network coding), but I don’t think we should be basing the future of the economics of the system on undemonstrated ideas. Without those (or even with), the result may be that miners self-limit the size of their blocks to propagate faster, but if this happens, larger, better-connected, and more centrally-located groups of miners gain a competitive advantage by being able to produce larger blocks. I would like to point out that there is nothing evil about this — a simple feedback to determine an optimal block size for an individual miner will result in larger blocks for better connected hash power. If we do not want miners to have this ability, “we” (as in: those using full nodes) should demand limitations that prevent it. One such limitation is a block size limit (whatever it is).
Ability to use a full node.
Skewed incentives for improvements… without actual pressure to work on these, I doubt much will change. Increasing the size of blocks now will simply make it cheap enough to continue business as usual for a while — while forcing a massive cost increase (and not just a monetary one) on the entire ecosystem.
Fees and long-term incentives.
I don’t think 1 MB is optimal. Block size is a compromise between scalability of transactions and verifiability of the system. A system with 10 transactions per day that is verifiable by a pocket calculator is not useful, as it would only serve a few large bank’s settlements. A system which can deal with every coffee bought on the planet, but requires a Google-scale data center to verify is also not useful, as it would be trivially out-competed by a VISA-like design. The usefulness needs in a balance, and there is no optimal choice for everyone. We can choose where that balance lies, but we must accept that this is done as a trade-off, and that that trade-off will have costs such as hardware costs, decreasing anonymity, less independence, smaller target audience for people able to fully validate, …
Choose wisely.

Mike Hearn responded:

this list is not a good place for making progress or reaching decisions.
if Bitcoin continues on its current growth trends it will run out of capacity, almost certainly by some time next year. What we need to see right now is leadership and a plan, that fits in the available time window.
I no longer believe this community can reach consensus on anything protocol related.
When the money supply eventually dwindles I doubt it will be fee pressure that funds mining
What I don’t see from you yet is a specific and credible plan that fits within the next 12 months and which allows Bitcoin to keep growing.

Peter Todd then pointed out that, contrary to Mike’s claims, developer consensus had been achieved within Core plenty of times recently. Btc-drak asked Mike to “explain where the 12 months timeframe comes from?”

Jorge Timón wrote an incredibly prescient reply to Mike:

We’ve successfully reached consensus for several softfork proposals already. I agree with others that hardfork need to be uncontroversial and there should be consensus about them. If you have other ideas for the criteria for hardfork deployment all I’m ears. I just hope that by “What we need to see right now is leadership” you don’t mean something like “when Gaving and Mike agree it’s enough to deploy a hardfork” when you go from vague to concrete.
Oh, so your answer to “bitcoin will eventually need to live on fees and we would like to know more about how it will look like then” it’s “no bitcoin long term it’s broken long term but that’s far away in the future so let’s just worry about the present”. I agree that it’s hard to predict that future, but having some competition for block space would actually help us get more data on a similar situation to be able to predict that future better. What you want to avoid at all cost (the block size actually being used), I see as the best opportunity we have to look into the future.
this is my plan: we wait 12 months… and start having full blocks and people having to wait 2 blocks for their transactions to be confirmed some times. That would be the beginning of a true “fee market”, something that Gavin used to say was his #1 priority not so long ago (which seems contradictory with his current efforts to avoid that from happening). Having a true fee market seems clearly an advantage. What are supposedly disastrous negative parts of this plan that make an alternative plan (ie: increasing the block size) so necessary and obvious. I think the advocates of the size increase are failing to explain the disadvantages of maintaining the current size. It feels like the explanation are missing because it should be somehow obvious how the sky will burn if we don’t increase the block size soon. But, well, it is not obvious to me, so please elaborate on why having a fee market (instead of just an price estimator for a market that doesn’t even really exist) would be a disaster.

Some suspected Gavin/Mike were trying to rush the hard fork for personal reasons.

Mike Hearn’s response was to demand a “leader” who could unilaterally steer the Bitcoin project and make decisions unchecked:

No. What I meant is that someone (theoretically Wladimir) needs to make a clear decision. If that decision is “Bitcoin Core will wait and watch the fireworks when blocks get full”, that would be showing leadership
I will write more on the topic of what will happen if we hit the block size limit… I don’t believe we will get any useful data out of such an event. I’ve seen distributed systems run out of capacity before. What will happen instead is technological failure followed by rapid user abandonment…
we need to hear something like that from Wladimir, or whoever has the final say around here.

Jorge Timón responded:

it is true that “universally uncontroversial” (which is what I think the requirement should be for hard forks) is a vague qualifier that’s not formally defined anywhere. I guess we should only consider rational arguments. You cannot just nack something without further explanation. If his explanation was “I will change my mind after we increase block size”, I guess the community should say “then we will just ignore your nack because it makes no sense”. In the same way, when people use fallacies (purposely or not) we must expose that and say “this fallacy doesn’t count as an argument”. But yeah, it would probably be good to define better what constitutes a “sensible objection” or something. That doesn’t seem simple though.
it seems that some people would like to see that happening before the subsidies are low (not necessarily null), while other people are fine waiting for that but don’t want to ever be close to the scale limits anytime soon. I would also like to know for how long we need to prioritize short term adoption in this way. As others have said, if the answer is “forever, adoption is always the most important thing” then we will end up with an improved version of Visa. But yeah, this is progress, I’ll wait for your more detailed description of the tragedies that will follow hitting the block limits, assuming for now that it will happen in 12 months. My previous answer to the nervous “we will hit the block limits in 12 months if we don’t do anything” was “not sure about 12 months, but whatever, great, I’m waiting for that to observe how fees get affected”. But it should have been a question “what’s wrong with hitting the block limits in 12 months?”

Mike Hearn again asserted the need for a leader:

There must be a single decision maker for any given codebase.

Bryan Bishop attempted to explain why this did not make sense with git architecture.

Finally, Gavin announced his intent to merge the patch into Bitcoin XT to bypass the peer review he had received on the bitcoin-dev mailing list.

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