I’m very excited about the recent Bitcoin Core v0.15 release. This is the first major release that I’ve been involved in from start to finish, and there are some great new features and improvements. Over the next few days, I’ll present a series of articles highlighting five of the most interesting changes to look out for.

I’m going to dive quite deep into the technical details of these changes since I think they’re all interesting and instructive. If you’re not interested in those details and just want a high-level overview of what’s new, then the release notes are a better place to look.

We’ll start by getting under the hood and look at an implementation change. Users shouldn’t see any changes in behaviour from this change, except for their Bitcoin Core client performing much better.

Per-output chainstate db (PR 10195)

Per-output chainstate db is a fantastic win in Initial Block Download time and general performance, as well as being a huge improvement in code simplicity. It also removes a potential DOS vector where an attacker could exhaust a node’s memory with carefully constructed transactions. Users shouldn’t see any functional difference, but it gives such a performance improvement that it’s worth looking at in more detail.

First, a bit of blockchain theory and history. The blockchain is simply an ordered log of the transactions that have been accepted by the network up to certain time. Once the blocks have been validated, that blockchain data is no longer useful to the node. What we’re actually interested in is the UTXO set — the set of all transaction outputs that have not yet been spent. All full nodes must keep a copy of this set so they can verify that transactions are spending coins that actually exist and that the signatures for those transactions are valid. In Bitcoin Core, that UTXO set is stored in a database called the chainstate db. Ideally a node would store this entirely in memory for fast lookups, but the chainstate can be flushed to disk if the node doesn’t have enough RAM to store the entire set.

The original Bitcoin software stored the entire block tree data, transactions and spends in a database called blkindex. Transactions would never get removed from that database, even when all of the outputs of those transactions had been spent. That’s fine up to a certain point, but it’s not scalable. Transactions grow linearly over time, and as the database grows, the time it takes to seek all the outputs for a new block in the database also grows. The result would be that the database would continue expanding and block validation and propagation would slow down more and more over time. If we’d kept using this model, all full nodes would be required to keep the full blockchain (roughly 130GB at the time of writing), and block validation would be really slow.

The first major change to the model came in v0.8, which included ultraprune. Ultraprune split up blkindex into a blocks database (for storing the historical blockchain), and a chainstate database (for storing transactions and outputs). The chainstate database was structured per-transaction. That meant that the main entry in the database was a list, containing all the outputs of a given transaction. As soon as all the outputs from a transaction had been spent, the entire transaction could be pruned from the chainstate database. This split also made block pruning possible, which was implemented in v0.11. The result of those changes together meant that today it’s possible to run a full validating node with only 4–5GB of disk space.

That’s the historical context. So what’s changed in v0.15? The chainstate database has been changed to be structured per-output instead of per-transaction. In practice that means:

It’s much faster to read and write individual outputs from the database, since we don’t need to read and write all of the containing transaction’s unspent outputs, just the output that we’re interested in. That means that validating a block is faster.

Memory usage is much smoother and can’t be blown up when reading/writing large transactions from the database. That means that we can use the chainstate cache more efficiently and flush to disk less often.

The code is much simpler, and we can make future improvements to flushing the chainstate database to disk.

There’s one small drawback to this:

The chainstate database becomes slightly larger. The reason the chainstate was initially implemented per-transaction was that it saved disk space. Instead of storing the transaction id once for each output, it only needed to be stored once across all the outputs in a transaction. Now, we need to store the txid once per output. This is partially mitigated by compression in the database layer which can avoid writing the same txid multiple times. In practice, the chainstate database will be about 15% larger with this change.

So what’s the bottom line for users? There are some performance results in the Pull Request:

Initial Block Download (IBD) and reindex is 30–40% faster

IBD and reindex use 10–20% less memory

IBD flushes to disk far less frequently.

30–40% speed-up in IBD is an enormous win. It means that when you start up a fresh node, you’ll catch up to the chain tip much more quickly.

Thanks to Pieter Wuille, Matt Corallo and Jimmy Song for input and feedback.

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