Removes Accounts Delta Hash

proposals/0223-removes-accounts-delta-hash.md

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+---
+simd: '0223'
+title: Removes Accounts Delta Hash
+authors:
+  - Brooks Prumo
+category: Standard
+type: Core
+status: Review
+created: 2025-01-13
+feature: LTdLt9Ycbyoipz5fLysCi1NnDnASsZfmJLJXts5ZxZz
+development:
+  - Agave: In development
+---
+
+## Summary
+
+With the introduction of the **Accounts Lattice Hash** (SIMD-215), the
+merkle-based **Accounts Delta Hash** is redundant and can be removed.
+
+
+## Motivation
+
+This SIMD follows on from SIMD-215, and shares its motivation:
+
+> The main goal is to scale Solana to billions accounts
+
+When a block is done being processed, it is frozen.  The bank hash is
+calculated during the freezing process, and includes calculating the **Accounts
+Delta Hash**.  The **Accounts Delta Hash** is the merkle-based hash of the
+accounts modified in this block.  As established in SIMD-215, merkle-based
+hashing of accounts hinders scaling.  And since SIMD-215 added the **Accounts
+Lattice Hash**, which is a hash of the total account state, the merkle-based
+hash of only the accounts modified per block is redundant.  Therefore the
+**Accounts Delta Hash** can be removed from the bank hash entirely.
+
+
+## New Terminology
+
+None.
+
+
+## Detailed Design
+
+Remove the **Accounts Delta Hash** from the **Bank Hash**.
+
+
+## Alternatives Considered
+
+None.
+
+
+## Impact
+
+Only validators will be impacted, and their performance will improve since the
+merkle-based hashing of modified accounts per block will no longer be
+performed.
+
+
+## Security Considerations
+
+None.
+
+
+## Backwards Compatibility
+
+Incompatible. This changes the bank hash, thus changing consensus.