Re: Configuring synchronous replication

On Thu, 2010-09-23 at 12:02 +0300, Heikki Linnakangas wrote:
> On 23/09/10 11:34, Csaba Nagy wrote:
> > In the meantime our DBs are not able to keep in sync via WAL
> > replication, that would need some kind of parallel WAL restore on the
> > slave I guess, or I'm not able to configure it properly - in any case
> > now we use slony which is working.
>
> It would be interesting to debug that case a bit more. Was bottlenecked
> by CPU or I/O, or network capacity perhaps?

Unfortunately it was quite long time ago we last tried, and I don't
remember exactly what was bottlenecked. Our application is quite
write-intensive, the ratio of writes to reads which actually reaches the
disk is about 50-200% (according to the disk stats - yes, sometimes we
write more to the disk than we read, probably due to the relatively
large RAM installed). If I remember correctly, the standby was about the
same regarding IO/CPU power as the master, but it was not able to
process the WAL files as fast as they were coming in, which excludes at
least the network as a bottleneck. What I actually suppose happens is
that the one single process applying the WAL on the slave is not able to
match the full IO the master is able to do with all it's processors.

If you're interested, I could try to set up another try, but it would be
on 8.3.7 (that's what we still run). On 9.x would be also interesting,
but that would be a test system and I can't possibly get there the load
we have on production...

Cheers,
Csaba.