Re: Freezing without write I/O

On 2013-06-07 20:10:55 +0100, Simon Riggs wrote:
> On 7 June 2013 19:56, Heikki Linnakangas <hlinnakangas@vmware.com> wrote:
> > On 07.06.2013 21:33, Simon Riggs wrote:
> >>
> >> Now that I consider Greg's line of thought, the idea we focused on
> >> here was about avoiding freezing. But Greg makes me think that we may
> >> also wish to look at allowing queries to run longer than one epoch as
> >> well, if the epoch wrap time is likely to come down substantially.
> >>
> >> To do that I think we'll need to hold epoch for relfrozenxid as well,
> >> amongst other things.
> 
> > The biggest problem I see with that is that if a snapshot can be older than
> > 2 billion XIDs, it must be possible to store XIDs on the same page that are
> > more than 2 billion XIDs apart. All the discussed schemes where we store the
> > epoch at the page level, either explicitly or derived from the LSN, rely on
> > the fact that it's not currently necessary to do that. Currently, when one
> > XID on a page is older than 2 billion XIDs, that old XID can always be
> > replaced with FrozenXid, because there cannot be a snapshot old enough to
> > not see it.
> 
> It does seem that there are two problems: avoiding freezing AND long
> running queries
> 
> The long running query problem hasn't ever been looked at, it seems,
> until here and now.

I'd say that's because it's prohibitive to run so long transactions
anyway since it causes too much unremovable bloat. 2bio transactions
really is a quite a bit, I don't think it's a relevant restriction. Yet.

Let's discuss it if we have solved the other problems ;)

Greetings,

Andres Freund

-- Andres Freund                       http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training &
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