Re: Checkpoints vs restartpoints

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.

Cheers,

Jeff