Re: getting rid of freezing

On 2013-05-23 22:09:02 -0400, Robert Haas wrote:
> On Thu, May 23, 2013 at 1:51 PM, Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
> > So, what I propose instead is basically:
> > 1) only vacuum non-all-visible pages, even when doing it for
> > anti-wraparound
>
> Check. We might want an option to force a scan of the whole relation.

Yea, thought of that as well. VACUUM (DEEP) ;).

> > 3) When we cannot mark a page all-visible or we cannot get the cleanup
> > lock, remember the oldest xmin on that page. We could set all visible
> > in the former case, but we want the page to be cleaned up sometime
> > soonish.

> I think you mean "in the latter case" not "in the former case". If
> not, then I'm confused.

Uh. Yes.

> > We don't even necessarily need to log the hint bits for all items since
> > the redo for all_visible could make sure all items are hinted. The only
> > problem is knowing up to where we can truncate pg_clog...

> [all-visible cannot restore hint bits without FPI because of torn pages]

I haven't yet thought about this sufficiently yet. I think we might have
a chance of working around this, let me ponder a bit.

But even if that means needing a full page write via the usual mechanism
for all visible if any hint bits needed to be set we are still out far
ahead of the current state imo.
* cleanup would quite possibly do an FPI shortly after in vacuum
anyway. If we do it for all visible, it possibly does not need to be
done for it.
* freezing would FPI almost guaranteedly since we do it so much
later.
* Not having to rescan the whole heap will be a bigger cost saving...

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
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