Re: Freezing without write I/O

On 2013-05-30 14:39:46 -0400, Robert Haas wrote:
> > Since we're not storing 64-bit wide XIDs on every tuple, we'd still need to
> > replace the XIDs with FrozenXid whenever the difference between the smallest
> > and largest XID on a page exceeds 2^31. But that would only happen when
> > you're updating the page, in which case the page is dirtied anyway, so it
> > wouldn't cause any extra I/O.
>
> It would cause some extra WAL activity, but it wouldn't dirty the page
> an extra time.

You probably could do it similarly to how we currently do
XLOG_HEAP_ALL_VISIBLE_CLEARED and just recheck the page on replay. The
insert/update/delete record will already contain a FPI if necessary, so
that should be safe.

> > This would also be the first step in allowing the clog to grow larger than 2
> > billion transactions, eliminating the need for anti-wraparound freezing
> > altogether. You'd still want to truncate the clog eventually, but it would
> > be nice to not be pressed against the wall with "run vacuum freeze now, or
> > the system will shut down".

> Interesting. That seems like a major advantage.

Hm. Why? If freezing gets notably cheaper I don't really see much need
for keeping that much clog around? If we still run into anti-wraparound
areas, there has to be some major operational problem.

Greetings,

Andres Freund

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