Re: Inconsistent DB data in Streaming Replication

On 2013-04-08 19:26:33 +0300, Ants Aasma wrote:
> On Mon, Apr 8, 2013 at 6:50 PM, Shaun Thomas  wrote:
> > On 04/08/2013 05:34 AM, Samrat Revagade wrote:
> >
> >> One solution to avoid this situation is have the master send WAL
> >> records to standby and wait for ACK from standby committing WAL files
> >> to disk and only after that commit data page related to this
> >> transaction on master.
> >
> >
> > Isn't this basically what synchronous replication does in PG 9.1+?
> 
> Not exactly. Sync-rep ensures that commit success is not sent to the
> client before a synchronous replica acks the commit record. What
> Samrat is proposing here is that WAL is not flushed to the OS before
> it is acked by a synchronous replica so recovery won't go past the
> timeline change made in failover, making it necessary to take a new
> base backup to resync with the new master. I seem to remember this
> being discussed when sync rep was committed. I don't recall if the
> idea was discarded only on performance grounds or whether there were
> other issues too.

Thats not going to work for a fair number of reasons:
* wal is streamed *from disk* not from memory
* what if the local node crashes/restarts immediately? Then the standby is farther ahead than the master.
* the performance implications of never writing data before flushing it are pretty severe
* ...

So this doesn't seem to solve anything.

Greetings,

Andres Freund

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