From Dencun to Fusaka: Ethereum's Next Leap

Author: stacymuur

Translation: Baihua Blockchain

Introduction: Why Fusaka is So Important Now

Ethereum continues to evolve to support more users, more transactions, and more applications while maintaining security and decentralization. Each upgrade addresses a bottleneck in the system. In early 2024, the Dencun upgrade introduced "blobs" (a new data format), providing a new way for Layer 2 (L2) rollups to store data cheaply on Ethereum. This is a significant milestone, but the demand for blobs quickly reached its limit, leading to congestion and rising fees once again.

Today, Ethereum launched the Fusaka upgrade on the Holesky testnet as the next step forward.

The headline feature of Fusaka is PeerData Availability Sampling (PeerDAS), a method for securely scaling blob throughput that allows nodes to verify data without downloading the entire blob. In addition to PeerDAS, Fusaka also improves gas rules, cryptographic tools, and the developer experience. For users, this means Ethereum becomes faster, cheaper, and easier to use, especially for rollups that millions rely on daily.

By launching on Holesky before the mainnet, Fusaka provides developers, validators, and application teams the opportunity to prepare for the next chapter of Ethereum.

From Dencun to Fusaka

To understand the importance of Fusaka, it helps to look back. In early 2024, Ethereum released the Dencun upgrade, introducing blobs as a new data format. Blobs allow rollups to publish bundled data to Ethereum in a cost-effective and secure manner. This breakthrough significantly reduced user fees on networks like Optimism, Arbitrum, and zkSync.

However, the demand for blobs grew so rapidly that rollups could not consistently secure blob space, causing fees to spike again. Ethereum needed a way to further scale blob throughput without overwhelming regular nodes. This is exactly what Fusaka, launched today on Holesky, provides.

Understanding PeerDAS

At the core of Fusaka is PeerDAS (Peer Data Availability Sampling, EIP-7594), a new way for Ethereum nodes to check the actual availability of blob data.

Problems Before Fusaka

Until today, nodes had to download the entire blob even if they only needed to confirm its existence. This is secure but inefficient.

Imagine a library where each member must read every book to verify that one is on the shelf. As the number of books increases, this effort becomes unsustainable. Similarly, as more blobs are added to Ethereum blocks, nodes become overwhelmed under unnecessary data pressure. This limits blob throughput and drives up fees for rollup users during high-demand periods.

How PeerDAS Works

PeerDAS uses erasure coding (a mathematical method that splits data into many small pieces). Imagine tearing a massive book into hundreds of chapters—just a random small subset can prove the existence of the entire book.

Nodes no longer download the entire blob but sample a few pieces from their peer nodes. If enough nodes confirm their random samples, the network can guarantee with high probability that the entire blob is available.

It's like a book club where each member randomly checks two or three chapters. If all samples agree, the whole group can be confident that the entire book is intact without anyone having to read it all.

Scalable Design: Blob Parameter Only Fork

Ethereum will not significantly increase blob capacity all at once. Fusaka introduces the Blob Parameter Only (BPO) fork (EIP-7892), a method to gradually increase blob limits after PeerDAS is activated.

On Holesky, the rollout plan is as follows:

October 1, 2025 – Fusaka activates at 08:48 UTC.
October 7, 2025 – BPO1 raises the blob target from 6 to 10, and the maximum from 9 to 15.
October 13, 2025 – BPO2 raises the target to 14 and the maximum to 21.

This incremental approach ensures that performance can be measured at each step and gives node operators time to adapt their hardware. Unlike a sudden leap, Ethereum scales in a controlled and safer manner.

Beyond Blobs: Strengthening Ethereum's Layer 1

Fusaka is not just about blobs. It also improves Ethereum's Layer 1 foundation:

Gas Rules: The default block gas limit is raised to 60 million (EIP-7935), while individual transactions are capped at approximately 16.7 million gas (EIP-7825). This prevents giant transactions from crowding out others and prepares for future parallel execution.
Cryptography: Optimizations for modular exponentiation (EIP-7883, 7823) improve the pricing of complex mathematical operations. A new precompile (EIP-7951) provides native support for P-256 signatures, widely used for passkeys and device-level security.
Network: Removal of legacy proof-of-stake fields (EIP-7642) saves bandwidth and simplifies client code.
Block Encoding: Introduction of a block size limit (EIP-7934) prevents extreme blocks from slowing down propagation.

These changes collectively enhance Ethereum's resilience and efficiency as activity scales.

Building with Fusaka

Ethereum upgrades also focus on usability. Fusaka introduces features that make life easier for developers and safer for users:

Passkeys: With native support for P-256 signatures, wallets can offer passkey login directly on Ethereum (for iPhone, Android devices, and browsers).
Bitwise Operations: The new CLZ opcode (EIP-7939) reduces the cost of compression methods and zero-knowledge (zk) circuits.
Transparency: Deterministic proposer foresight (EIP-7917) makes the block proposer schedule known in advance, enabling transaction pre-confirmation.
Predictability: Blob fee guarantees (EIP-7918) keep blob fees relative to execution fees, ensuring stable economics. For developers, these are powerful new tools. For users, they translate into smoother applications, cheaper zk protocols, and more predictable fees.

Testnet Rollout: Holesky Today, Mainnet Coming Soon

Ethereum upgrades always go through testnets before reaching the mainnet. The rollout of Fusaka is phased:

Holesky: Launched on October 1, 2025. BPO1 and BPO2 will follow within two weeks. Holesky will also be retired afterward, marking both a launch and a farewell.
Sepolia: Scheduled for October 14, 2025.
Hoodi: Scheduled for October 28, 2025.

The mainnet activation will only be scheduled after all three testnets successfully upgrade, currently expected in December 2025.

Real-World Changes

These technical upgrades will extend to real-world benefits:

Rollup Users: During transaction peaks, blob slots would quickly fill up, leading to rising fees. With PeerDAS and higher blob capacity, fees will remain more stable.
Node Operators: Validators upgrading today on Holesky are now compatible with PeerDAS and the new gas rules. Those who do not upgrade will fork off the chain.
Application Developers: Wallet teams can support passkey login cheaply and natively.
zk Developers: Protocols can use the CLZ opcode to reduce the proof costs of zk circuits.

Security and Governance

Ethereum strikes a balance between innovation and caution. In addition to Fusaka, a bug bounty program has been launched, offering rewards of up to $2 million to encourage testing before the mainnet.

Fusaka also showcases Ethereum's governance model: developers propose Ethereum Improvement Proposals (EIPs), client teams implement them, and node operators decide by upgrading their software. While Fusaka is expected to be non-controversial, the final decision always rests with the community.

Outlook: Ethereum After Fusaka

Ethereum's roadmap unfolds in phases: Dencun, Pectra, Fusaka, and further into the future. Each upgrade clears a bottleneck and paves the way for the next step. The significance of Fusaka lies in its secure scaling of blob throughput, enabling rollups to handle more transactions while maintaining reasonable node requirements.

For users, this means a cheaper and more reliable L2 experience.
For developers, it unlocks tools like passkey authentication and zk efficiency.
For validators, it proves that Ethereum can scale while maintaining decentralization.

Conclusion

The Fusaka upgrade launched today on Holesky is not just another name on the roadmap. It marks the beginning of expanding blob capacity beyond current limits, driven by PeerDAS and secured through phased forks. It also strengthens Ethereum's foundation in areas like gas, cryptography, and networking.

As Fusaka rolls out from Holesky to Sepolia and Hoodi, then to the mainnet, it exemplifies Ethereum's philosophy: cautious evolution, inclusive progress, and a focus on long-term usability. For millions of users, developers, and validators worldwide, Fusaka is a concrete step toward a faster, cheaper, and more practical Ethereum.

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