In-depth essays · #2 / 50 · Architecture, Consensus & Upgrades
Inside EIP-4844 Blobs: The Temporary Data Containers That Slashed L2 Fees by 90%
How moving from permanent calldata to 18-day expiring blob space reshaped rollup economics
Before the March 2024 Dencun upgrade, swapping a token on an Ethereum L2 often cost anywhere from fifty cents to several dollars. Immediately after the upgrade, the same swap dropped below a penny. The engine behind that dramatic drop is the ‘blob’—a dedicated sidecar data container attached to Ethereum blocks.
1. Why L2 Fees Used to Be High: The Cost of Permanent Calldata
To inherit Ethereum’s security, a rollup must publish compressed batches of its L2 transactions to Ethereum L1. Prior to Dencun, rollups stuffed those batches into transaction calldata. Because every full Ethereum node must store calldata on disk forever, writing bytes to calldata is intentionally expensive.
In practice, 80% to 90% of a user’s L2 transaction fee was not paying for L2 computation at all—it was paying rent to store raw batch data permanently across every Ethereum node worldwide.
2. Why 18 Days (4,096 Epochs) Is Enough for Rollup Security
Why does a rollup publish raw batch data to L1 in the first place? For an optimistic rollup, verifiers need the data during the roughly 7-day challenge window to recompute state transitions and submit a fault proof if needed. For a ZK rollup, nodes need to verify that the state transition proven by the validity proof has publicly available inputs so nobody’s balance is locked in a black box.
Once that window passes, consensus nodes no longer need to keep the raw 128 KB data payloads forever. By guaranteeing availability for 4,096 epochs (about 18 days) before pruning, EIP-4844 blobs deliver full rollup security at a fraction of the storage cost.
3. A Separate Fee Market for Data: Multidimensional Gas Pricing
EIP-4844 also introduced a separate fee market for blobs. Previously, when an L1 NFT mint or token frenzy spiked mainnet execution gas, L2 users saw their fees jump too because L1 transactions and L2 calldata competed for the same gas limit.
Blobs now have their own independent base fee that adjusts block by block depending on how many blobs are used relative to the target. Subsequent upgrades—Pectra and Fusaka—expanded blob capacity further, keeping L2 data posting costs near negligible levels during normal traffic.
4. Spotting Blob Trucks in Ethereum City
In Ethereum City, orange blob trucks drive across the bridges from L2 districts into downtown Ethereum whenever rollups post data batches. The ‘Blobs per block’ and ‘Blob fee’ indicators in the Traffic tab read live EIP-4844 fields directly from mainnet blocks.
When blob fees sit at a few wei, L2 districts are settling batches for fractions of a cent; when a market storm fills every blob slot across consecutive blocks, the rising blob fee appears immediately in the city’s traffic board and the next morning’s newspaper.
Related districts in the city
Related glossary terms
This essay is an explainer written to help you read the city; it is not investment advice. Facts on this page are as of 2026-10 and may change. If you spot a mistake, please tell us through the contact page.