The Dencun Upgrade Explained: Ethereum's Blob Transaction Era

Dencun is the name for the Ethereum network upgrade activated in March 2024 that combined execution-layer changes (code-named "Cancun") and consensus-layer changes (code-named "Deneb"), most notably introducing blob-carrying transactions through EIP-4844. Its headline effect was a sharp reduction in the cost of posting data from layer 2 rollups back to Ethereum, which in turn lowered transaction fees for users of those networks.

What problem Dencun was built to solve

Before Dencun, rollups — the layer 2 networks discussed in /blog/layer2-guide — posted their transaction data to Ethereum as regular "calldata," a type of permanent, expensive on-chain storage. Rollups needed this data on layer 1 so that anyone could reconstruct and verify their state, a core requirement of the /blog/rollup-centric-roadmap-explained that Ethereum has pursued instead of scaling layer 1 directly. But calldata's cost was driven by the same /glossary/gas market as every other Ethereum transaction, meaning rollup costs rose and fell with mainnet congestion, undercutting the whole point of moving activity off layer 1.

The core fix: EIP-4844 and blob transactions

EIP-4844 introduced a new transaction type carrying "blobs" — large chunks of data attached to a transaction but not processed by the /glossary/smart-contract execution environment (the EVM) at all. This distinction matters: because blobs aren't run through contract logic, nodes don't need to store them permanently — they're deleted after roughly 18 days, once rollups have had ample time to use them. The mechanics of this new data type are covered in more depth in /blog/what-is-a-blob-transaction.

Blobs are also priced through their own separate fee market, with a base fee that adjusts based on blob-space demand independently of regular transaction gas prices. This means a surge in ordinary Ethereum activity doesn't necessarily spike rollup data costs, and vice versa.

Immediate effect on layer 2 fees

The practical result was significant: rollups that adopted blob transactions saw the cost of posting data to Ethereum drop substantially compared to using calldata, and that savings was largely passed through to end users as lower transaction fees on those networks. This made using /defi applications and other on-chain activity on major rollups noticeably cheaper than it had been on layer 1 or on rollups still using pre-Dencun calldata.

Aspect	Before Dencun (calldata)	After Dencun (blobs)
Storage duration	Permanent	Temporary (roughly 18 days)
Fee market	Shared with all L1 gas	Separate blob fee market
Cost sensitivity	Tied to general L1 congestion	Tied to blob-space demand specifically
Effect on rollup fees	Higher, more volatile	Substantially lower, more stable

Other components bundled into Dencun

While EIP-4844 was the headline feature, Dencun bundled several other EIPs affecting things like /glossary/smart-contract storage access patterns and validator operations. This is typical of Ethereum upgrades — a scheduled hard fork, as described in /blog/how-ethereum-hard-forks-work, bundles multiple related and unrelated improvements that happened to be ready around the same time, rather than shipping one change in isolation.

What Dencun did not change

Dencun did not reduce base layer-1 transaction fees for users transacting directly on Ethereum mainnet rather than through a rollup. It also didn't eliminate the need for rollups to eventually post data somewhere — it simply made the mechanism for doing so cheaper and more purpose-built. Users still choosing to interact directly with Ethereum mainnet, rather than an L2, continue to pay ordinary gas fees calculated as described in /blog/what-is-gas-limit-vs-gas-price.

How rollups actually adopted blob transactions

Dencun's fee savings weren't automatic for every rollup on day one — each rollup's own software needed to be updated to actually construct and submit blob transactions instead of continuing to use calldata. Major rollups moved to adopt this fairly quickly given the scale of the cost savings involved, but the pace of migration varied, meaning the realized fee reduction for users depended partly on which specific rollup they were using and how quickly that rollup's operators shipped the necessary software changes. This is a useful reminder that a base-layer upgrade like Dencun sets new capabilities in motion, but the applications and networks built on top still need to build support for those capabilities before users see the benefit.

Measuring the actual impact

Because blob data is tracked separately from regular gas usage, the effect of Dencun can be observed directly by comparing a rollup's per-transaction cost before and after adoption, or by tracking the total blob-space usage across the network over time on a block explorer. In the months following activation, data consistently showed rollup transaction costs dropping substantially compared to pre-Dencun levels, though the exact magnitude varied by rollup and by how much blob-space demand grew as more usage shifted toward the cheaper option — an example of the general dynamic described in /blog/what-is-eip-1559-base-fee-burn, where a separate fee market responds to its own specific demand.

Why this mattered for the broader roadmap

Dencun is widely viewed as the first major concrete step toward Ethereum's longer-term "danksharding" vision, which aims to scale blob capacity substantially further over time. Each subsequent upgrade is expected to incrementally increase the number of blobs a block can carry, continuing to lower the marginal cost of rollup data as demand grows.

Bottom line

Dencun was the Ethereum upgrade that introduced blob transactions via EIP-4844, giving rollups a purpose-built, temporary, and separately priced way to post data to layer 1. Its main practical effect was substantially cheaper transactions on major rollups, reinforcing Ethereum's strategy of scaling through layer 2s rather than expanding layer-1 capacity directly.