Re: Sync Rep: First Thoughts on Code

On Thu, 2008-12-18 at 12:08 +0900, Fujii Masao wrote:

> >> Agreed, I also think that hard code is better. But I'm nervous that "off"
> >> keeps us waiting for replication in cases other than DDL, e.g. flush
> >> buffer, truncate clog, checkpoint.. etc. synchronous_replication = off
> >> is quite similar to synchronous_commit = off. If we would hard code #4,
> >> the performance might degrade although it's asynchronous replication.
> >> So, I'd like to hard code #3. What is your opinion?
> >
> > We don't do that when we flush buffer, truncate clog or checkpoint, not
> > sure why you mention those.
> >
> > We ForceSyncCommit when we
> > * VACUUM FULL
> > * CREATE/DROP DATABASE or USER
> > * Create/Drop Tablespace
> >
> > I don't see a problem in forcing an fsync for those. I will sleep safer
> > knowing those guys are on disk even in async mode.
> 
> If my understanding is correct, XLOG flush is forced up to buffer's LSN
> when flushing buffer even if asynchronous commit case. Am I missing
> something?

Yes, please check the call points for ForceSyncCommit.

Do I think every xlog flush should be synchronous, no, I don't. That's
why we have a user settable parameter for it.

-- Simon Riggs           www.2ndQuadrant.comPostgreSQL Training, Services and Support