Re: Hot standby and removing VACUUM FULL

On Wed, 2009-11-25 at 03:12 +0000, Greg Stark wrote:
> On Wed, Nov 25, 2009 at 2:10 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> > As long as there's not anything the master actually does differently
> > then I can't see where there'd be any performance testing to do. What's
> > bothering me about this is that it seems likely that we'll find places
> > where the master has to do things differently. I'd rather we made the
> > status visible; if we get through a release cycle without needing to
> > check it, we can always take the function out again. But if we don't,
> > and then find out midway through the 8.5 release cycle that we need to
> > be able to check it, things could be a bit sticky.
>
> Well the only thing that's been discussed is having vacuum require a
> minimum age before considering a transaction visible to all to reduce
> the chance of conflicts on cleanup records. But that would require an
> actual tunable, not just a flag. And it's something that could
> conceivably be desirable even if you're not running a HS setup (if
> someone ever reimplements time travel for example).

I will add this also, if it looks simple to do so. Even if we yank it
out later better to have the code for discussion purposes than just a
conceptual bikeshed.

> So I'm not sure adding a flag before there's an actual need for it is
> necessarily going to be helpful. It may turn out to be insufficient
> even if we have a flag.

Same situation as in archiving.

The debate was eventually carried that we should have
archive_mode = on
archive_xxxx = NNNN for additional parameters

> And then there's the question of what the slave should do if the
> master was running without the flag. Do we make it throw an error?

Well, it can't even enter HS mode, so no error needed.

> Does that mean the master needs to insert information to that effect
> in the wal logs? What if you shut down the master switch the flag and
> start it up again and you had a standby reading those logs all along.
> Will it be able to switch to HS mode now? We won't know until we know
> why this flag was necessary and what change in behaviour it might have
> caused.

I'm more comfortable running a new machine when it has an "off" switch.

--
Simon Riggs www.2ndQuadrant.com