Re: Visibility map thoughts

On Mon, 2007-11-05 at 22:45 +0000, Heikki Linnakangas wrote:
> > 1) Do as you say above. What are some of the cost trade-offs here? It
> > seems that frequent VACUUM FREEZE runs would keep the visibility map
> > mostly full, but will also cause more writing. I suppose the worst case
> > is that every tuple write needs results in two data page writes, one
> > normal write and another to freeze it later, which sounds bad. Maybe
> > there's a way to try to freeze the tuples on a page before it's written
> > out?
>
> It would also create more WAL traffic, because freezing tuples needs to
> be WAL-logged.

The thought crossed my mind, but I couldn't think of any reason that
would need to be logged. Of course you're right, and the comments
explain it well.

> 5) Have a more fine-grain equivalent of relfrozenxid. For example one
> frozenxid per visibility map page, so that whenever you update the
> visibility map, you also update the frozenxid. To advance the
> relfrozenxid in pg_class, you scan the visibility map and set
> relfrozenxid to the smallest frozenxid. Unlike relfrozenxid, it could be
> set to FrozenXid if the group of pages are totally frozen.
>

Wouldn't that still require WAL traffic? Otherwise how can you guarantee
that the FrozenXid hits disk before TruncateCLOG truncates the old xmin
away?

Regards,
Jeff Davis