ethdb: improve docs and add IdealBatchSize

ethdb/interface.go

@@ -16,10 +16,16 @@
 
 package ethdb
 
+// Code using batches should try to add this much data to the batch.
+// The value was determined empirically.
+const IdealBatchSize = 100 * 1024
+
+// Putter wraps the database write operation supported by both batches and regular databases.
 type Putter interface {
 	Put(key []byte, value []byte) error
 }
 
+// Database wraps all database operations. All methods are safe for concurrent use.
 type Database interface {
 	Putter
 	Get(key []byte) ([]byte, error)
@@ -29,6 +35,8 @@ type Database interface {
 	NewBatch() Batch
 }
 
+// Batch is a write-only database that commits changes to its host database
+// when Write is called. Batch cannot be used concurrently.
 type Batch interface {
 	Putter
 	Write() error