A blob in Ethereum is a large chunk of transaction data that gets attached to a special kind of transaction and is kept only for a short time. Blobs give Layer 2 rollups a cheaper place to publish their data by using space that is separate from the usual transaction data on the main chain.
Blobs arrived with Ethereum’s EIP-4844, also called Proto-Danksharding. The idea is to prepare Ethereum for full sharding by adding a way to handle temporary data at scale. Instead of packing everything into permanent block space, EIP-4844 introduces blob space so networks can move more data through Ethereum without overloading it. This reduces pressure on normal block space while keeping the network’s security model intact and setting the stage for future scaling steps.
A blob cannot appear on its own. It rides along with a blob-carrying transaction that includes two extra fields not found in regular Ethereum transactions: a maximum fee the sender is willing to pay per “blob gas” and a list of versioned hashes that reference the blobs included. Validators use those references to fetch and verify the actual blob data. This flow lets rollups post big batches of user transactions to Ethereum at lower cost, because the heavy data sits in blob space rather than permanent storage.
What makes blobs different from CALLDATA
Before EIP-4844, rollups posted their data as CALLDATA. That path is pricey because CALLDATA is processed by the EVM and stored forever. Blobs live in their own space, are not executed by the EVM, and are priced separately. This keeps rollup data cheaper and avoids pushing up fees for normal users transacting on the main chain.
Consensus nodes store full blob contents only for a limited period, long enough for others to verify the rollup’s state. After that window, nodes discard the bytes to save disk space. What remains on chain is the commitment to the data, so verifiers can still check that the rollup made its data available at the time of inclusion.
Blobs use a separate fee market from regular transactions. Prices adjust with supply and demand for blob space, similar in spirit to gas pricing, but isolated so spikes in rollup data demand do not directly raise costs for everyday transactions.
To help decentralization and keep blocks manageable, Ethereum limits how many blobs fit in a block. Documentation for EIP-4844 and builder guides commonly describe an upper bound per block, and the blob-carrying transaction format is what enables those limits to be enforced on chain.
Role in Layer 2 rollups
Rollups bundle thousands of transactions into a blob, publish the commitment via a blob-carrying transaction, and let the network hold the raw bytes for a short time so anyone can reconstruct and verify the rollup’s state. This approach lowers the L1 posting costs for rollups and improves throughput for users.
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