On Fri, 2011-03-18 at 17:08 -0400, Aidan Van Dyk wrote:
> On Fri, Mar 18, 2011 at 3:41 PM, Markus Wanner <markus@bluegap.ch> wrote:
> > On 03/18/2011 08:29 PM, Simon Riggs wrote:
> >> We could do that easily enough, actually, if we wished.
> >>
> >> Do we wish?
> >
> > I personally don't see any problem letting a standby show a snapshot
> > before the master. I'd consider it unneeded network traffic. But then
> > again, I'm completely biased.
>
> In fact, we *need* to have standbys show a snapshot before the master.
>
> By the time the master acks the commit to the client, the snapshot
> must be visible to all client connected to both the master and the
> syncronous slave.
>
> Even with just a single server postgresql cluster, other
> clients(backends) can see the commit before the commiting client
> receives the ACK. Just that on a single server, the time period for
> that is small.
>
> Sync rep increases that time period by the length of time from when
> the slave reaches the commit point in the WAL stream to when it's ack
> of that point get's back to the wal sender. Ideally, that ACK time is
> small.
>
> Adding another round trip in there just for a "go almost to $COMIT,
> ok, now go to $COMMIT" type of WAL/ack is going to be pessimal for
> performance, and still not improve the *guarentees* it can make.
>
> It can only slightly reduce, but not eliminated that window where them
> master has WAL that the slave doesn't, and without a complete
> elimination (where you just switch the problem to be the slave has the
> data that the master doesn't), you haven't changed any of the
> guarantees sync rep can make (or not).
Well explained observation. Agreed.
-- Simon Riggs http://www.2ndQuadrant.com/books/PostgreSQL Development, 24x7 Support, Training and Services
