Re: Checkpoints vs restartpoints

On Tue, Jun 9, 2015 at 05:20:23PM -0700, Jeff Janes wrote:
> On Tue, Jun 9, 2015 at 4:20 PM, Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>
> wrote:
>
> Hi
>
> Why do standby servers not simply treat every checkpoint as a
> restartpoint?  As I understand it, setting checkpoint_timeout and
> checkpoint_segments higher on a standby server effectively instruct
> standby servers to skip some checkpoints.  Even with the same settings
> on both servers, the server could still choose to skip a checkpoint
> near the checkpoint_timeout limit due to the vagaries of time keeping
> (though I suppose it's very unlikely).  But what could the advantage
> of skipping checkpoints be?  Do people deliberately set hot standby
> machines up like this to trade a longer crash recover time for lower
> write IO?
>
>
> When a hot standby server is initially being set up using a rather old base
> backup and an archive directory, it could be applying WAL at a very high rate
> such that it would replay master checkpoints multiple times a second (when the
> master has long periods with little write activity and has checkpoints driven
> by timeouts during those periods).  Actually doing restartpoints that often
> could be annoying.  Presumably there would be few dirty buffers to write out,
> since each checkpoint saw little activity, but you would still have to circle
> the shared_buffers twice, and fsync whichever files did happen to get some
> changes.

Ah, so even thought standbys don't have to write WAL, they are fsyncing
shared buffers. Where is the restart point recorded, in pg_controldata?
c
--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

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