Re: Hot standby, recent changes

On Mon, 2009-12-07 at 10:02 +0200, Heikki Linnakangas wrote:
> Simon Riggs wrote:
> > On Sun, 2009-12-06 at 17:26 -0500, Robert Haas wrote:
> >
> >> For what it's worth, this doesn't seem particularly unlikely or
> >> unusual to me.
> >
> > I don't know many people who shutdown both nodes of a highly available
> > application at the same time. If they did, I wouldn't expect them to
> > complain they couldn't run queries on the standby when an two obvious
> > and simple workarounds exist to allow them to access their data: start
> > the master again, or make the standby switchover, both of which are part
> > of standard operating procedures.
>
> It might not be common or expected in a typical HA installation, but it
> would be a very strange limitation in my mind. It might well happen e.g
> in a standby used for reporting, or when you do PITR.

Yes, its possible that the standby can be shutdown while disconnected
from a master. If that occurs, all we are saying is that we cannot use
shutdown checkpoints as starting points. If the primary was set up in
default mode, then wal_standby_info = on and so there will be
running_xact WAL records immediately after each checkpoint to use as
starting points. We don't need shutdown checkpoints as well.

So adding shutdown checkpoints as a valid starting place does not help
in this case, it just makes the code harder to test.

> > It doesn't seem very high up the list of additional features, at least.
>
> Well, it's in the patch now. I'm just asking you to not break it.

There's been many things in the patch that have been removed. This is by
far the least important of the things removed in the name of getting
this done as soon as possible, with good quality. I see no reason for
your eagerness to include this feature. I have removed it; as you say,
its in the repo if someone wants to add it in later.

--
Simon Riggs www.2ndQuadrant.com