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Comment by gibybo | Hacker News Reader

This is the size of the blockchain over time: https://blockchain.info/charts/blocks-size. It's currently 12 GB and has a maximum growth rate of ~6 MB/hour. The hope is that storage and bandwidth get cheaper faster than the blockchain gets bigger.

There are definitely challenges with its current growth rate. As currently designed, it's capped at about 25,000 transactions per hour. The developers are working on addressing this limitation, but the solution is complex and they aren't done yet. We went from ~200 transactions/hour in Jan 2012 to ~2,700 transactions/hour in Dec 2013, so it's conceivable we end up getting pretty close to (or hitting) the cap in the near future.

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Does that mean someone can denial-of-service all of bitcoin just by moving a balance between a couple wallets repeatedly? Can one person (or group)'s activity crowd out other people so their transactions can't be added to the blockchain? What prevents that from happening?

25,000/hour is a shockingly small number to me. I feel like I must be misunderstanding something, or else someone from 4chan would have already DOS'd bitcoin for fun.

There already is a transaction fee - miners will ignore your transaction if you pay too little or nothing.

Transactions normally include a small fee, to incentivize miners and disincentivize abuse. You don't have to pay the fee, but if there are more transactions being broadcast than the blockchain can accommodate, the fee-paying ones get priority. (It's a little more complicated than that: when you generate a transaction, you choose whatever fee you want, and they get prioritized based on a combination of the fee, the transaction size in KB, and how long it's been since those coins were last moved.)

Current transaction fees are typically in the range of 0.1-0.5mBTC, so in order to continuously flood out everyone else, you'd need to spend more than 2.5BTC/hour in fees.

It is definitely a small number. Visa is capable of handling 80 million+ per hour, for example. However, transactions are prioritized by transaction fee so you would have to pay a fee higher than all other transactions to block them out. Currently this would cost you something like $100k per day, but if you started to do this you would rapidly increase the cost of transactions, so the cost to perform this attack would grow much more expensive.

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