> ## Documentation Index
> Fetch the complete documentation index at: https://docs.optimism.io/llms.txt
> Use this file to discover all available pages before exploring further.

# How the DA footprint block limit works

> Understand the Data Availability (DA) footprint block limit introduced in the Jovian hardfork, how the DA footprint is calculated, and why the default gas scalar is 400.

The **Data Availability (DA) Footprint Block Limit** was introduced in the **[Jovian hardfork](https://docs.optimism.io/notices/upgrade-17)** to limit the total amount of transaction data that can fit into a block based on a scaled estimate of the compressed size (or "data availability footprint") of that data.
This page explains the problem the limit solves, how the DA footprint is calculated, and how the `daFootprintGasScalar` parameter shapes the limit.
To change the scalar on your chain, follow the [DA footprint setup guide](/chain-operators/guides/features/setting-da-footprint).

## Why a DA footprint limit exists

When an OP Stack chain receives more calldata-heavy transactions than can fit into the L1's available blob space, the [batcher can throttle the chain's throughput](https://docs.optimism.io/chain-operators/guides/configuration/batcher#batcher-sequencer-throttling).

However, continuous batcher throttling may cause the base fee to drop to the [minimum base fee](https://docs.optimism.io/chain-operators/guides/features/setting-min-base-fee),
causing unnecessary losses for the chain operator and negative user experiences such as priority fee auctions.
And without throttling, the batcher runs the risk of becoming overwhelmed with chain data to batch to the blob space.
Limiting the amount of (estimated compressed) calldata taken up by transactions in a block using their total DA footprint can reduce the need for batcher throttling and its related issues.

## How the DA footprint is calculated

For all [non-deposit transactions](https://docs.optimism.io/reference/glossary#deposited-transaction) processed by an OP Stack chain, a DA footprint value is recorded alongside the transaction's gas usage.
The DA footprint can be configured via the `daFootprintGasScalar` variable in the `SystemConfig` contract on the L1 chain.
The DA footprint is automatically calculated for every transaction first by calculating a `daUsageEstimate` for that transaction:

```python theme={null}
daUsageEstimate = max(
          minTransactionSize,
          (intercept + fastlzCoef * tx.fastlzSize) // 1e6
      )
```

where the `minTransactionSize`, `intercept`, `fastlzCoef`, and `tx.fastlzSize` are as specified in the [Fjord specs](https://specs.optimism.io/protocol/fjord/exec-engine.html#fees).
Then the `daUsageEstimate` is multiplied by the `daFootprintGasScalar` to get the `daFootprint` for that individual transaction.

```python theme={null}
      daFootprint += daUsageEstimate * daFootprintGasScalar
```

The `daFootprint` for all the transactions in a block are then added together to calculate that block's total `daFootprint`.
With the block's total `daFootprint` calculated:

<Info>
  * The block's total `daFootprint` must stay below its `gasLimit`.
  * The `blobGasUsed` property of each block header is set to that block's `daFootprint`.
  * The base fee update calculation then uses `gasMetered := max(gasUsed, blobGasUsed)` as a replacement for the `gasUsed` variable.
</Info>

From Jovian, transaction receipts also record the transaction's DA Footprint in the receipt's `blobGasUsed` field, as well as the block's `daFootprintGasScalar` in a new field with the same name.

## What the DA footprint gas scalar controls

The *DA footprint gas scalar* scales the *estimated DA usage in bytes* to the gas dimension, and this scaled estimate of the DA usage is what we call the *DA footprint*.
This allows us to limit the estimated DA usage using the block's `gasLimit`: the effective limit of estimated DA usage per block is `gasLimit / daFootprintGasScalar` bytes.
So *increasing* this scalar makes DA usage more gas-heavy, so *decreases* the limit, and vice versa.

This is closely related to how the calldata (floor) cost of `40` gas per non-zero byte limits the total amount of calldata in a block.
A DA footprint gas scalar of `400` effectively limits *incompressible* calldata by a factor of `10` compared to its limit without a DA footprint block limit.
As such, the feature can be seen as an extension of the existing calldata limit.
But instead of repricing the calldata (floor) gas cost, the limit is accounted in parallel to EVM execution gas, and is based on the more relevant FastLZ-based DA usage estimate instead of simply counting zero and non-zero bytes.

## Why the default value is 400

The default scalar of `400` was chosen so that it protects chains in worst-case DA spam scenarios, but has negligible to no impact during normal operation.
Careful [analyses](https://github.com/ethereum-optimism/design-docs/blob/main/protocol/da-footprint-block-limit.md) have been done to estimate the impact on current OP Stack chains and pick the right default.
Only high-throughput chains would occasionally even see a small impact from the resulting DA footprint limit (as slightly faster rising base fees). The DA footprint limit is mostly invisible.
On mid to low-throughput chains, the feature is expected to have no impact under normal usage conditions.
It acts more like an insurance to protect against worst-case *incompressible* DA spam.

<Info>
  A `daFootprintGasScalar` value of `0` in the `SystemConfig` is treated as the default of `400`. To effectively disable the limit, set the scalar to a very low value such as `1`.
</Info>

## Next steps

* To set or change the scalar on your chain, follow [Set the DA Footprint Gas Scalar](/chain-operators/guides/features/setting-da-footprint).
* For the full set of fee-related `SystemConfig` parameters, see the [fee parameters reference](/chain-operators/reference/fee-parameters).

## References

* [DA Footprint Configuration Spec](https://specs.optimism.io/protocol/jovian/system-config.html#da-footprint-configuration)
* [Jovian Upgrade Spec](https://specs.optimism.io/protocol/jovian/overview.html)
* [SystemConfig Contract Spec](https://specs.optimism.io/protocol/system-config.html)
* [Design Doc](https://github.com/ethereum-optimism/design-docs/blob/main/protocol/da-footprint-block-limit.md)