The Bitcoin block size is an ongoing topic of discussion among the community as a main component of the Bitcoin protocol that sets a limit on the number of transactions within each block. The controversy around the debate is because some people believe that a limit should not be imposed or should be significantly higher than 1 MB.
The Bitcoin block size is not 1 MB, as after the implementation of SegWit which “compresses” transactions, the size can occasionally reach to 1.6 MB and more. The final block size depends on how many transactions have their signature components segregated out of the base block. The more SegWit transactions occur, the more transactions a block can include.
This blog post answers a common question: Why developers assess the block size and what are the reasons they do not agree to increase the size? For example, why they do not agree towards an increase to 1 GB? Why is such an increase not beneficial for the network? Let’s see…
The Answer: Bandwidth
When a bitcoin node runs, every block receives and transmits block data to its peers. Each node tends to connect to 8 peers. Therefore, a node simultaneously receives and transmits block information to 8 other nodes. Consequently, if a node connects to 8 peers the energy usage is as follows, given a 1 MB block size:
- 1 MB for a block received approximately every 10 minutes on average.
- 8 MB for block information sent to 8 other peers.
If the block size increases to 100 MB:
- 100 MB for a block received approximately every 10 minutes on average.
- 800 MB for block information sent to 8 other peers.
As a result, the bandwidth costs rise exponentially to unsustainable levels, depending on the block size increase. Furthermore, if the bandwidth is low, the process would take a lot of time and cause latency. Receiving and transmitting blocks would not be a simple and fast procedure. In the case above, a 100 MB block would take 10 times more to be transmitted than a 1 MB block via the same connection.
There is an additional issue that concerns new nodes wishing to participate to the network. As all new blocks add up on top of the genesis block, a new node needs to dedicate a lot of resources to synchronize with the rest of the network. Currently the blockchain size is around 385 GB. New nodes need to verify the whole blockchain, starting from the genesis block. Even with the current blockchain size, this process can take days or weeks if the bandwidth is low.
In addition, the problem applies to nodes that are providing information to the new nodes. As you can imagine, transmitting 385 GB of data is extremely time consuming and costly. When a node transmits this data to multiple new nodes, this translates to TB of outgoing data, therefore hundreds of dollars per month.
Some Bitcoin forks attempted to increase their block size, with the most notable one being Bitcoin Cash. The attempt was to increase the block size to 32 MB. A further fork on the Bitcoin Cash network – Bitcoin SV (BSV) – aims to increase the block size to 1 TB. Currently, very few people run nodes of these forks in comparison to Bitcoin. Since running a node on such a network is going to be expensive in the long run, these networks will enhance a higher degree of centralization. Technically, such networks would also be more vulnerable to network attacks such as DDOS attacks.
What’s next for the Bitcoin block size?
Theoretically SegWit could allow 4 MB blocks. However, a block consisting of only signature data is not realistic. In a scenario of a full adoption of SegWit, the blocks could reach a size of close to 2 MB. We have seen a rapid increase to SegWit adoption since June 2021. The adoption rate rose from 53% to 83%.
It seems that SegWit is currently the most important improvement in terms of the block size controversy to date. It has managed to tackle (to some degree) the cost and settlement issues of the blockchain size that keeps increasing. The bandwidth costs and time constraints will probably be always the main aspect that developers examine when improvement proposals are discussed among the community.
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