postgresql - reusing old master as slave in PostgreSQL

Question

I have a Master node (A) and a slave node (B).

Assume a checkpoint has occurred at instant T and some transactions have been completed since the checkpoint (The dirty pages are not yet persisted on the disk). Assume the WAL position after the transaction is X.

Now node A fails, and I have promoted B as master.

I want to repair the node A and attach it as slave to B (the current master).

My doubt here is, will the transactions that occurred in node A (old master node A) get replayed and then it replays the streaming wal from current master (B) ??

My questions are,

1) In this case, from which point the wal will be streamed from new master to new slave? From the checkpoint or from position X?

2) when a old master is reused as slave, from where does the replay start? Any older wal present in the node gets replayed?

Answer 1

如果节点 A 领先于节点 B，则需要使用pg_rewind将其回退到可以用作 B 的备用节点的状态。

它所做的是确定最后一个公共检查点并替换此后更改的所有块。