core: journal the snapshot inside leveldb, not a flat file

core/rawdb/accessors_snapshot.go

@@ -95,3 +95,26 @@ func DeleteStorageSnapshot(db ethdb.KeyValueWriter, accountHash, storageHash com
 func IterateStorageSnapshots(db ethdb.Iteratee, accountHash common.Hash) ethdb.Iterator {
 	return db.NewIteratorWithPrefix(storageSnapshotsKey(accountHash))
 }
+
+// ReadSnapshotJournal retrieves the serialized in-memory diff layers saved at
+// the last shutdown. The blob is expected to be max a few 10s of megabytes.
+func ReadSnapshotJournal(db ethdb.KeyValueReader) []byte {
+	data, _ := db.Get(snapshotJournalKey)
+	return data
+}
+
+// WriteSnapshotJournal stores the serialized in-memory diff layers to save at
+// shutdown. The blob is expected to be max a few 10s of megabytes.
+func WriteSnapshotJournal(db ethdb.KeyValueWriter, journal []byte) {
+	if err := db.Put(snapshotJournalKey, journal); err != nil {
+		log.Crit("Failed to store snapshot journal", "err", err)
+	}
+}
+
+// DeleteSnapshotJournal deletes the serialized in-memory diff layers saved at
+// the last shutdown
+func DeleteSnapshotJournal(db ethdb.KeyValueWriter) {
+	if err := db.Delete(snapshotJournalKey); err != nil {
+		log.Crit("Failed to remove snapshot journal", "err", err)
+	}
+}

core/rawdb/schema.go

@@ -41,9 +41,12 @@ var (
 	// fastTrieProgressKey tracks the number of trie entries imported during fast sync.
 	fastTrieProgressKey = []byte("TrieSync")
 
-	// snapshotRootKey tracks the number and hash of the last snapshot.
+	// snapshotRootKey tracks the hash of the last snapshot.
 	snapshotRootKey = []byte("SnapshotRoot")
 
+	// snapshotJournalKey tracks the in-memory diff layers across restarts.
+	snapshotJournalKey = []byte("SnapshotJournal")
+
 	// Data item prefixes (use single byte to avoid mixing data types, avoid `i`, used for indexes).
 	headerPrefix       = []byte("h") // headerPrefix + num (uint64 big endian) + hash -> header
 	headerTDSuffix     = []byte("t") // headerPrefix + num (uint64 big endian) + hash + headerTDSuffix -> td