questions about LMDB

Hi list , Recently I read docs about lmdb , there are two sentences 1) readers do not block writers 2) writers do not block readers I can understand 'readers do not block writers' , but cannot understand the second one , can someone help explain , how lmdb achieve 'writers do not block readers', below is my understandings , please correct me if anything wrong. if the access pattern is write - read, since lmdb only support two version of data , when the write has been started but not committed , the concurrent read may read stale data since write has not been committed. Cheers, xinxin