Could the 1MB block size limit be replaced with a growth-dependent limit? From what I have read (here, for example) the 1MB limit is to prevent spamming of the blockchain. And if/when Bitcoin becomes more widely adopted we will have to increase the limit so that the network can handle more transactions per second.

Instead, could we have a rule like "blocks can be no larger than 1.5 times the average of the last 100 blocks"? The hope is that we could automate this spam prevention measure while still allowing for growth. Has this idea been considered anywhere else at all?

Maybe this be too large of a protocol change. Would we be able to get everyone to update their nodes to follow the slightly altered protocol?



Technically, this is possible. However, it would be a hardfork, so all users would have to upgrade. In such a large ecosystem that exists around Bitcoin today, this would be rather disrupting.

Such change would also have unclear security implications. Miners would be naturally eager to accept as many transactions as possible, so a spammer may gradually push block size limit up to increase the time it takes the network to process new blocks. This would lead to an increase in orphan rate, which would make it easier to mount a 51% attack, especially with the help of selfish mining.

This idea was discussed many times on Bitcointalk. While most of the developers agree that a change would be necessary eventually, any specific plans have yet to be announced.

By the way, some altcoins, such as Monero and Cryptonite, have variable block size limit using rules similar to the ones you propose.

  • Which altcoins use a similar rule? – morsecoder Sep 27 '14 at 14:07
  • A hardfork would be disruptive, but it would also be extremely disruptive if the bitcoin network can't handle the amount of transactions on it. And a spammer could gradually push the block size limit up, but they would spend all their money doing it. It would have to be a purely malicious spammer. Such a spammer could also be pushing the block size up to the full 1MB every block right now, but no one is. My point is that I don't think that is very likely to happen. – morsecoder Sep 27 '14 at 14:34
  • And having larger blocks at all will increase the orphan rate. We are going to need larger blocks eventually, so there's no getting around that. – morsecoder Sep 27 '14 at 14:35
  • This would also probably be one of the easier hardforks to do because those who do update and those who don't will likely still remain compatible for some time (since the block size is typically still fairly far under the 1MB limit). – morsecoder Sep 27 '14 at 14:37
  • @StephenM347 Monero is an example of altcoin which has block size limit dependent on the sizes of previous blocks. As for Bitcoin, this question is a subject of heated discussions, but AFAIK currently there is no plans to change anything. – abacabadabacaba Sep 28 '14 at 0:48

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