On 10/06/2010 08:31 AM, Heikki Linnakangas wrote:
> On 06.10.2010 01:14, Josh Berkus wrote:
>> Last I checked, our goal with synch standby was to increase availablity,
>> not decrease it.
>
> No. Synchronous replication does not help with availability. It allows
> you to achieve zero data loss, ie. if the master dies, you are
> guaranteed that any transaction that was acknowledged as committed, is
> still committed.
Strictly speaking, it even reduces availability. Which is why nobody
actually wants *only* synchronous replication. Instead they use quorum
commit or semi-synchronous (shudder) replication, which only requires
*some* nodes to be in sync, but effectively replicates asynchronously to
the others.
From that point of view, the requirement of having one synch and two
async standbies is pretty much the same as having three synch standbies
with a quorum commit of 1. (Except for additional availability of the
later variant, because in case of a failure of the one sync standby, any
of the others can take over without admin intervention).
Regards
Markus Wanner