bitcoin/doc/release-notes-31384.md

• Node and Mining

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• PR #31384 fixed an issue where block reserved weight for fixed-size block header, transactions count, and coinbase transaction was done in two separate places. Before this pull request, the policy default for the maximum block weight was 3,996,000 WU, calculated by subtracting 4,000 WU from the 4,000,000 WU consensus limit to account for the fixed-size block header, transactions count, and coinbase transaction. During block assembly, Bitcoin Core clamped custom -blockmaxweight value to not be more than the policy default.

  Additionally, the mining code added another 4,000 WU to the initial reservation, reducing the effective block template size to 3,992,000 WU.

  Due to this issue, the total reserved weight was always 8,000 WU, meaning that even when specifying a -blockmaxweight higher than the policy default, the actual block size never exceeded 3,992,000 WU.

  The fix consolidates the reservation into a single place and introduces a new startup option, -blockreservedweight (default: 8,000 WU). This startup option specifies the reserved weight for the fixed-size block header, transactions count, and coinbase transaction. The default value of -blockreservedweight was chosen to preserve the previous behavior.

  Upgrade Note: The default -blockreservedweight ensures backward compatibility for users who relied on the previous behavior.

  Users who manually set -blockmaxweight to its maximum value of 4,000,000 WU should be aware that this value previously had no effect since it was clamped to 3,996,000 WU.

  Users lowering -blockreservedweight should ensure that the total weight (for the block header, transaction count, and coinbase transaction) does not exceed the reduced value.

  As a safety check, Bitcoin core will fail to start when -blockreservedweight init parameter value is lower than 2000 weight units.

  Bitcoin Core will also fail to start if the -blockmaxweight or -blockreservedweight init parameter exceeds consensus limit of 4,000,000 weight units.