分离为只读/写数据库

Many people said that one can achieve higher performance by creating read-only and write-only database, be frankly, I can't fully understand it. Some people told me that writing will require many different locks, which will slow down the reading...but as my understanding, many reading in a system doesn't require locks, such as oracle consistent read, how do the locks affect reading? Besides if i want to shard database, does read/writing separation offer more value? Many people said that one can achieve higher pe