Dencun gave rollups cheaper, temporary data space on Ethereum

Inherited archive entry · retained from the earlier edition. Its factual, legal and policy claims have not all been reverified for 19 September 2026. Research scope.

A cheaper, separate lane for rollup data

Ethereum activated the Dencun upgrade at epoch 269568 on 13 March 2024, according to the mainnet announcement, which describes its central feature as 'ephemeral data blobs' intended to 'help reduce L2 transaction fees'. That feature is EIP-4844, commonly called proto-danksharding. Before this upgrade, a rollup that needed to publish its transaction data to Ethereum for verification had to use calldata, a format the execution layer processes and stores indefinitely, and prices accordingly.

What a blob is and how it is priced

A blob holds roughly 128 kilobytes of data that, per the EIP, 'cannot be accessed by EVM execution, but whose commitment can be accessed'. Blobs are priced through their own gas market, adjusting by a formula the specification compares to EIP-1559: a base fee per blob that rises or falls depending on whether recent blocks exceeded a target of three blobs, up to a hard cap of six. Because this market is separate from ordinary execution gas, a surge in blob demand does not, by itself, raise the cost of an unrelated ether transfer, and the reverse also holds.

The saving depends on the data being temporary

The specification sets blob retention at 4,096 epochs, which it states is 'around 18 days'; after that, nodes may prune the data. Ethereum's roadmap page, as retrieved on 16 September 2026, explains the trade this makes possible: rollups only need the underlying data available long enough for a challenge or a proof to be checked, not forever, so the protocol does not ask every node to store it permanently. A rollup that wants its transaction history retrievable after 18 days must arrange that separately, through its own operator, a third-party data-availability service, or by continuing to accept the cost of calldata for the data it judges worth permanent storage. Dencun made a specific class of cost cheaper; it did not make Ethereum a permanent public archive of rollup history, and a chain's claim to be fully secured by Ethereum should be read against that limit.

Questions to carry forward

• Does a given rollup rely on blobs alone for data availability, or does it also retain data through its own infrastructure?
• What happens to a user's ability to reconstruct history if the blob window has passed and the rollup's own archive is unavailable?
• Has blob demand approached the six-per-block cap, and what would that mean for costs during congestion?

The mechanism separates the question of what is cheap from the question of what is kept. Cheaper is verifiable in the fee data; kept is a separate promise that has to be checked against what each rollup actually does once the blob window closes.