From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Andres Freund <andres@2ndquadrant.com>
Cc: Robert Haas <robertmhaas@gmail.com>
Cc: Bruce Momjian <bruce@momjian.us>
Cc: Heikki Linnakangas <hlinnakangas@vmware.com>
Cc: Rajeev rastogi <rajeev.rastogi@huawei.com>
Cc: Ants Aasma <ants@cybertec.at>
Cc: PostgreSQL-development <pgsql-hackers@postgresql.org>
Cc: Markus Wanner <markus@bluegap.ch>
Subject: Re: Proposal for CSN based snapshots
Date: Thu, 15 May 2014 17:58:04 -0400
Message-ID: <20140515215803.GC7857@eldon.alvh.no-ip.org> (raw)
In-Reply-To: <20140515214100.GA12739@awork2.anarazel.de>
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<20140515183436.GD25053@momjian.us>
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<20140515200632.GB16014@awork2.anarazel.de>
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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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To: pgsql-hackers@postgresql.org
Cc: alvherre@2ndquadrant.com, andres@2ndquadrant.com, robertmhaas@gmail.com, bruce@momjian.us, hlinnakangas@vmware.com, rajeev.rastogi@huawei.com, ants@cybertec.at, markus@bluegap.ch
Subject: Re: Proposal for CSN based snapshots
In-Reply-To: <20140515215803.GC7857@eldon.alvh.no-ip.org>
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