Re: Sync Rep: First Thoughts on Code

On Thu, 2008-12-11 at 17:07 +0200, Heikki Linnakangas wrote:
> Simon Riggs wrote:
> > On Thu, 2008-12-11 at 09:27 -0500, Aidan Van Dyk wrote:
> > 
> >> But "catchup" *has* to be *done* before PostgreSQL can enter "sync rep".
> > 
> > Not true. Please reread the thread where Heikki questions that and I
> > reply. This was Fujii-san's idea, which I now agree with.
> 
> I think the confusion here is about what exactly "sync rep" means in 
> this situation. It's true that you can start streaming the WAL before 
> the standby has fully caught up. 

Yep.

> But from the client's point of view, 
> there's not much point in streaming the log *synchronously* and making 
> the client to wait for the acknowledment from the standby, if the 
> acknowledgment from the standby that WAL has be streamed up to point X, 
> doesn't actually guarantee that the slave can recover all the way to 
> that point.

I disagree. This morning I showed it was possible, given the
synchronisation I outlined.

There is a slight relaxation of that in the current proposal, so you
need to take that up if you see any problem there.

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