Re: New and interesting replication issues with 9.2.8 sync rep

On 2014-05-02 18:57:08 -0700, Josh Berkus wrote:
> Just got a report of a replication issue with 9.2.8 from a community member:
> 
> Here's the sequence:
> 
> 1) A --> B (sync rep)
> 
> 2) Shut down B
> 
> 3) Shut down A
> 
> 4) Start up B as a master
> 
> 5) Start up A as sync replica of B
> 
> 6) A successfully joins B as a sync replica, even though its transaction
> log is 1016 bytes *ahead* of B.
> 
> 7) Transactions written to B all hang
> 
> 8) Xlog on A is now corrupt, although the database itself is OK

This is fundamentally borked practice.

> Now, the above sequence happened because of the user misunderstanding
> what sync rep really means.  However, A should not have been able to
> connect with B in replication mode, especially in sync rep mode; that
> should have failed.  Any thoughts on why it didn't?

I'd guess that B, while starting up, has written further WAL records
bringing it further ahead of A.

Greetings,

Andres Freund

-- Andres Freund                       http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training &
Services