Bitcoin is based on a blockchain, which is a string of blocks, where each block includes numerous transactions. The total size of the blockchain is inexorably growing, and this has implications for the future of running Bitcoin nodes. Currently, the Bitcoin blockchain is 178.8 GB in size as of this writing on 14 August 2018, a size so large that it is difficult to download and a significant obstacle towards running a full Bitcoin node.
Catching up to block size
Each Bitcoin block originally had a hard cap of 1 MB but this was changed when Segregated Witness (SegWit) was implemented. Now, Bitcoin blocks can be up to 1.2 MB on average and rarely 2 MB or slightly more. On average, there is a new Bitcoin block every 10 minutes, which adds up to 144 MB of new data added to the blockchain every day. This equals almost exactly 1 GB of data being added to the blockchain every week, which is similar to the size of a movie download. Every month, 4 GB of data is added to the blockchain and every year this is 52.5 GB of data. These numbers assume an average of 1 MB per block, which may be an underestimate due to SegWit. However, empty and partially-filled blocks are a common occurrence, so choosing 1 MB is probably better than performing calculations with 1.2 MB.
Up until 2015, less than this amount was being added to the blockchain every year, since there was less data being used for transactions than the maximum capacity of blocks on average. However, since 2015, Bitcoin has been filling up blocks consistently, and this will likely continue since Bitcoin is becoming even more popular as time progresses.
There are many computers with 100 GB of data storage or less and for these computers, running a full Bitcoin node is already impossible. In order to run a full Bitcoin node and therefore make the Bitcoin network healthier by actively participating in broadcasting data to users, a computer must have a full and current version of the blockchain. Most high-end computers have 1 TB of storage, which is 1,000 GB, and these computers will be able to download the entire Bitcoin blockchain for more than a decade. Hard drive sizes on high-end computers will likely continue to increase, so there should always be a suitable computer for people wanting to run a Bitcoin node.
Fewer full nodes?
However, the biggest problem as the Bitcoin blockchain grows is the time it takes to download the whole blockchain. It already takes weeks and even months with a good internet connection, making it unfeasible for most Bitcoin users to download the Bitcoin blockchain and run a full node. Equivalently, this situation makes it unfeasible for most Bitcoin users to use the Bitcoin Core wallet software.
Currently there are 9,637 full Bitcoin nodes in the world, and this number has been declining since March 2018 from a peak near 10,500. This is a very small amount of nodes compared to the number of Bitcoin users in the world and in the futurem as Bitcoin’s blockchain grows, it will be progressively more difficult for the number of nodes to increase since the time to download the blockchain will get even longer.
Unfortunately, fewer nodes means the Bitcoin network is less healthy. It also means that there are less people running a full version of Bitcoin and choosing online wallets, which are far less secure and increases risk across the Bitcoin world. Due to the nature of Bitcoin, in that it requires a full blockchain history to function, this problem may only be fixable with faster internet and computers that have more storage.
Current mitigations include running a full node in “pruned” mode, currently allowing a storage space below 10 GB. Depending on the version and client, the Bitcoin blockchain could also be faster to download via torrenting.
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