| From: | Simon Riggs <simon(at)2ndQuadrant(dot)com> |
|---|---|
| To: | Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com> |
| Cc: | Markus Wanner <markus(at)bluegap(dot)ch>, Dimitri Fontaine <dimitri(at)2ndQuadrant(dot)fr>, Jeff Davis <pgsql(at)j-davis(dot)com>, Josh Berkus <josh(at)agliodbs(dot)com>, pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: Issues with Quorum Commit |
| Date: | 2010-10-08 08:25:01 |
| Message-ID: | 1286526301.2304.537.camel@ebony |
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| Lists: | pgsql-hackers |
On Fri, 2010-10-08 at 10:56 +0300, Heikki Linnakangas wrote:
> >
> > Or what kind of customers do you think really need a no-lag solution for
> > read-only queries? In the LAN case, the lag of async rep is negligible
> > and in the WAN case the latencies of sync rep are prohibitive.
>
> There is a very good use case for that particular set up, actually. If
> your hot standby is guaranteed to be up-to-date with any transaction
> that has been committed in the master, you can use the standby
> interchangeably with the master for read-only queries.
This is an important point. It is desirable, but there is no such thing.
We must not take any project decisions based upon that false premise.
Hot Standby is never guaranteed to be up-to-date with master. There is
no such thing as certainty that you have the same data as the master.
All sync rep gives you is a better durability guarantee that the changes
are safe. It doesn't guarantee those changes are transferred to all
nodes prior to making the data changes on any one standby.
--
Simon Riggs www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Training and Services
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