Re: Proposal for CSN based snapshots

From: Amit Langote <amitlangote09(at)gmail(dot)com>
To: Heikki Linnakangas <hlinnakangas(at)vmware(dot)com>
Cc: Greg Stark <stark(at)mit(dot)edu>, Robert Haas <robertmhaas(at)gmail(dot)com>, Andres Freund <andres(at)2ndquadrant(dot)com>, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Bruce Momjian <bruce(at)momjian(dot)us>, Rajeev rastogi <rajeev(dot)rastogi(at)huawei(dot)com>, Ants Aasma <ants(at)cybertec(dot)at>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>, Markus Wanner <markus(at)bluegap(dot)ch>
Subject: Re: Proposal for CSN based snapshots
Date: 2014-06-20 05:54:32
Message-ID: CA+HiwqHwEOdCZS=ykgb3iwfEaNNY0wHgs0yU3qHQQekeg96X4g@mail.gmail.com
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Hi,

On Fri, Jun 13, 2014 at 7:24 PM, Heikki Linnakangas
<hlinnakangas(at)vmware(dot)com> wrote:
> Yeah, that seems like a better design, after all.
>
> Attached is a new patch. It now keeps the current pg_clog unchanged, but
> adds a new pg_csnlog besides it. pg_csnlog is more similar to pg_subtrans
> than pg_clog: it's not WAL-logged, is reset at startup, and segments older
> than GlobalXmin can be truncated.
>
> This addresses the disk space consumption, and simplifies pg_upgrade.
>
> There are no other significant changes in this new version, so it's still
> very much WIP. But please take a look!
>

Thanks for working on this important patch. I know this patch is still
largely a WIP but I would like to report an observation.

I applied this patch and did a few pgbench runs with 32 clients (this
is on a not so powerful VM, by the way) . Perhaps you suspect such a
thing already but I observed a relatively larger percentage of time
being spent in XLogInsert().

Perhaps XLogCtlInsert.insertpos_lck contention via GetXLogInsertRecPtr()?

--
Amit

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