Merge pull request #3317 from Pana/doc/db

doc: add docs about db related crates

Author: Pana

docs/repo/layout.md

@@ -59,7 +59,20 @@ Other utility crates:
 
 ### dbs
 
-The [dbs](../../crates/dbs) directory contains the database crates.
+All database-related code is located under [dbs](../../crates/dbs), which contains the core logic for the entire node's data storage. Specifically, it mainly includes two major categories of data: blockchain (block header, block body, receipt) and state (state trie, storage trie), as well as some indexing and development data (trace).
+
+The underlying layer primarily uses rocksdb, and also uses sqlite to store some snapshot-related data.
+
+1. kvdb-rocksdb: Rust wrapper for rocksdb, which depends on third-party crates (kvdb, rocksdb) at the underlying layer
+2. db: Mainly provides the open_database method, which returns a db instance that can be used for data reading and writing
+3. db-errors: Database operation error definitions
+4. **storage**: The main implementation of the state database, including mpt, mpt snapshot and other logic, providing lower-level read/write interfaces `StateTrait`. This crate is the core code for state storage
+5. statedb: Simple wrapper around storage, providing higher-level read interfaces `StateDbExt`
+
+Additionally, the upper-level modules that call the db module mainly consist of two parts:
+
+1. crates/cfxcore/core/src/block_data_manager: Encapsulates all data read/write interfaces
+2. crates/execution/executor/src/state: The state object of the execution module, which essentially wraps StateDb and provides account state reading and updating interfaces for EVM.
 
 ### network