Snapshots use Accounts Lattice Hash

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.