Date: Thu, 15 May 2014 17:58:04 -0400
From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Andres Freund <andres@2ndquadrant.com>
Cc: Robert Haas <robertmhaas@gmail.com>, Bruce Momjian <bruce@momjian.us>,
 Heikki Linnakangas <hlinnakangas@vmware.com>,
 Rajeev rastogi <rajeev.rastogi@huawei.com>, Ants Aasma <ants@cybertec.at>,
 PostgreSQL-development <pgsql-hackers@postgresql.org>,
 Markus Wanner <markus@bluegap.ch>
Subject: Re: Proposal for CSN based snapshots
Message-ID: <20140515215803.GC7857@eldon.alvh.no-ip.org>
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Andres Freund wrote:
> 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?

I didn't say it did ...

-- 
Álvaro Herrera                http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services


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