Re: Configuring synchronous replication

On 17/09/10 15:56, Simon Riggs wrote:
> On Fri, 2010-09-17 at 13:41 +0300, Heikki Linnakangas wrote:
>> On 17/09/10 12:49, Simon Riggs wrote:
>>> Without performance tests to demonstrate "why", these do sound hard to
>>> understand. But we should note that DRBD offers recv ("B") and fsync
>>> ("C") as separate options. And Oracle implements all 3 of recv, fsync
>>> and apply. Neither of them describe those options so simply and easily
>>> as the way we are proposing with a 4 valued enum (with async as the
>>> fourth option).
>>>
>>> If we have only one option for sync_rep = 'on' which of recv | fsync |
>>> apply would it implement? You don't mention that. Which do you choose?
>>
>> You would choose between recv, fsync and apply in the slave, with a GUC.
>
> So you would have both registration on the master and parameter settings
> on the standby? I doubt you mean that, so possibly need more explanation
> there for me to understand what you mean and also why you would do that.

Yes, that's what I meant. No-one else seems to think that's a good idea :-).

>> I don't expect any meaningful differences in terms of performance
>> between any of the discussed options. The big question right now is...
>
> This is the critical point. Politely, I would observe that *You* do not
> think there is a meaningful difference. *I* do, and evidence suggests
> that both Oracle and DRBD think so too. So we differ on what the "big
> question" is here.

We must be talking about different things again. There's certainly big
differences in the different synchronization levels and configurations,
but I don't expect there to be big performance differences between
patches to implement those levels. Once we got rid of the polling loops,
I expect the network and disk latencies to dominate.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com