On 06.10.2010 11:49, Fujii Masao wrote:
> On Wed, Oct 6, 2010 at 5:17 PM, Heikki Linnakangas
> <heikki.linnakangas@enterprisedb.com> wrote:
>> Sure, but it's not the synchronous aspect that increases availability. It's
>> the replication aspect, and we already have that. Making the replication
>> synchronous allows zero data loss in case the master suddenly dies, but it
>> comes at the cost of availability.
>
> Yep. But I mean that the synchronous aspect is helpful to increase the
> availability of the system which requires no data loss. In asynchronous
> replication, when the master goes down, we have to salvage the missing
> WAL for the standby from the failed master to avoid data loss. This would
> take very long and decrease the availability of the system which doesn't
> accept any data loss. Since the synchronous doesn't require such a salvage,
> it can increase the availability of such a system.
In general, salvaging the WAL that was not sent to the standby yet is
outright impossible. You can't achieve zero data loss with asynchronous
replication at all.
> If we want only no data loss, we have only to implement the wait-forever
> option. But if we make consideration for the above-mentioned availability,
> the return-immediately option also would be required.
>
> In some (many, I think) cases, I think that we need to consider availability
> and no data loss together, and consider the balance of them.
If you need both, you need three servers as Simon pointed out earlier.
There is no way around that.
-- Heikki Linnakangas EnterpriseDB http://www.enterprisedb.com