SIMD-0220: Snapshots use Accounts Lattice Hash

proposals/0220-snapshots-use-accounts-lattice-hash.md

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+---
+simd: '0220'
+title: Snapshots use Accounts Lattice Hash
+authors:
+  - Brooks Prumo
+category: Standard
+type: Core
+status: Review
+created: 2025-01-08
+feature: LTsNAP8h1voEVVToMNBNqoiNQex4aqfUrbFhRH3mSQ2
+development:
+  - Agave: Implemented (v2.2.0)
+---
+
+## Summary
+
+Use the **Accounts Lattice Hash** as the **Snapshot Hash**, which enables
+removing the merkle-based accounts hash calculation.
+
+
+## Motivation
+
+This SIMD follows on from SIMD-215, and shares its motivation:
+
+> The main goal is to scale Solana to billions accounts
+
+When snapshots are taken, they contain a **Snapshot Hash**.   This hash is
+based on the merkle-based hash of the total account state.  As established in
+SIMD-215, this merkle-based hashing of all 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 all accounts used in the current
+**Snapshot Hash** is redundant work.  The **Snapshot Hash** can be updated to
+use the **Accounts Lattice Hash** instead.
+
+
+## New Terminology
+
+None.
+
+
+## Detailed Design
+
+When constructing the **Snapshot Hash**, the merkle-based hash of all accounts
+will be replaced by the **Accounts Lattice Hash**.  And since SIMD-215 removes
+the **Epoch Accounts Hash**, the **Snapshot Hash** becomes:
+
+```
+snapshot_hash := accounts_lattice_hash.out()
+```
+
+Specifically, the **Snapshot Hash** for slot `S` is the 32-byte blake3 of the
+**Accounts Lattice Hash** at slot `S`.
+
+Note that SIMD-215 is a prerequisite for this SIMD.
+
+
+## Alternatives Considered
+
+None.
+
+
+## Impact
+
+Only validators will be impacted, and their performance will improve since the
+merkle-based hashing of all accounts will no longer be performed.
+
+
+## Security Considerations
+
+None.
+
+
+## Backwards Compatibility
+
+Snapshots will have a new hash, so consumers of snapshots will need to know
+which method to use for hash validation.