The journal edit records are sent by the active name-node to the backup-node with RPC:
During the name-node throughput benchmark, the size of byte array records is around 8000. Then the serialization and deserialization is time-consuming. I wrote a simple application to test RPC with byte array parameter. When the size got to 8000, each RPC call need about 6 ms. While name-node sync 8k byte to local disk only need 0.3~0.4ms.