Re: Proposal for CSN based snapshots

On 2014-05-15 17:37:14 -0400, Alvaro Herrera wrote:
> Andres Freund wrote:
> > On 2014-05-15 15:40:06 -0400, Robert Haas wrote:
> > > On Thu, May 15, 2014 at 2:34 PM, Bruce Momjian <bruce@momjian.us> wrote:
> 
> > > If the larger clog size is a show-stopper (and I'm not sure I have an
> > > intelligent opinion on that just yet), one way to get around the
> > > problem would be to summarize CLOG entries after-the-fact.  Once an
> > > XID precedes the xmin of every snapshot, we don't need to know the
> > > commit LSN any more.  So we could read the old pg_clog files and write
> > > new summary files.  Since we don't need to care about subcommitted
> > > transactions either, we could get by with just 1 bit per transaction,
> > > 1 = committed, 0 = aborted.  Once we've written and fsync'd the
> > > summary files, we could throw away the original files.  That might
> > > leave us with a smaller pg_clog than what we have today.
> > 
> > I think the easiest way for now would be to have pg_clog with the same
> > format as today and a rangewise much smaller pg_csn storing the lsns
> > that are needed. That'll leave us with pg_upgrade'ability without
> > needing to rewrite pg_clog during the upgrade.
> 
> Err, we're proposing a patch to add timestamps to each commit,
> http://www.postgresql.org/message-id/20131022221600.GE4987@eldon.alvh.no-ip.org
> which does so in precisely this way.

I am not sure where my statements above conflict with committs?

Greetings,

Andres Freund

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