core, ethdb: polish code

core/headerchain.go

@@ -620,7 +620,8 @@ func (hc *HeaderChain) setHead(headBlock uint64, headTime uint64, updateFn Updat
 	// This step must be performed after synchronizing the key-value store;
 	// otherwise, in the event of a panic, it's theoretically possible to
 	// lose recent key-value store writes while the ancient store deletions
-	// remain, leading to data inconsistency.
+	// remain, leading to data inconsistency, e.g., the gap between the key
+	// value store and ancient can be created due to unclean shutdown.
 	if delFn != nil {
 		// Ignore the error here since light client won't hit this path
 		frozen, _ := hc.chainDb.Ancients()

ethdb/leveldb/leveldb.go

@@ -329,7 +329,8 @@ func (db *Database) Path() string {
 func (db *Database) Sync() error {
 	// In theory, the WAL (Write-Ahead Log) can be explicitly synchronized using
 	// a write operation with SYNC=true. However, there is no dedicated key reserved
-	// for this purpose, and even a nil key (key=nil) is considered a valid database entry.
+	// for this purpose, and even a nil key (key=nil) is considered a valid
+	// database entry.
 	//
 	// In LevelDB, writes are blocked until the data is written to the WAL, meaning
 	// recent writes won't be lost unless a power failure or system crash occurs.

ethdb/pebble/pebble.go

@@ -185,7 +185,15 @@ func New(file string, cache int, handles int, namespace string, readonly bool) (
 		fn:       file,
 		log:      logger,
 		quitChan: make(chan chan error),
-		writeOptions: &pebble.WriteOptions{Sync: false},
+
+		// Use asynchronous write mode by default. Otherwise, the overhead of frequent fsync
+		// operations can be significant, especially on platforms with slow fsync performance
+		// (e.g., macOS) or less capable SSDs.
+		//
+		// Note that enabling async writes means recent data may be lost in the event of an
+		// application-level panic (writes will also be lost on a machine-level failure,
+		// of course). Geth is expected to handle recovery from an unclean shutdown.
+		writeOptions: pebble.NoSync,
 	}
 	opt := &pebble.Options{
 		// Pebble has a single combined cache area and the write
@@ -426,12 +434,11 @@ func (d *Database) Path() string {
 // Sync flushes all pending writes in the write-ahead-log to disk, ensuring
 // data durability up to that point.
 func (d *Database) Sync() error {
-	b := d.db.NewBatch()
-
 	// The entry (value=nil) is not written to the database; it is only
 	// added to the WAL. Writing this special log entry in sync mode
 	// automatically flushes all previous writes, ensuring database
 	// durability up to this point.
+	b := d.db.NewBatch()
 	b.LogData(nil, nil)
 	return d.db.Apply(b, pebble.Sync)
 }

triedb/pathdb/buffer.go

@@ -135,8 +135,11 @@ func (b *buffer) flush(db ethdb.KeyValueStore, freezer ethdb.AncientWriter, node
 		start = time.Now()
 		batch = db.NewBatchWithSize(b.nodes.dbsize() * 11 / 10) // extra 10% for potential pebble internal stuff
 	)
-	// Explicitly sync the state freezer, ensuring that all written
+	// Explicitly sync the state freezer to ensure all written data is persisted to disk
-	// data is transferred to disk before updating the key-value store.
+	// before updating the key-value store.
+	//
+	// This step is crucial to guarantee that the corresponding state history remains
+	// available for state rollback.
 	if freezer != nil {
 		if err := freezer.SyncAncient(); err != nil {
 			return err