On 2016-04-06 10:04:42 +0100, Simon Riggs wrote: > On 6 April 2016 at 09:45, Andres Freund <andres@anarazel.de> wrote: > > > On 2016-04-06 09:18:54 +0100, Simon Riggs wrote: > > > Rather than take that option, I went to the trouble of writing a patch > > that > > > does the same thing but simpler, less invasive and more maintainable. > > > Primarily, I did that for you, to avoid you having wasted your time and > > to > > > allow you to backpatch a solution. > > > > But it doesn't. It doesn't solve the longstanding problem of checkpoints > > needlessly being repeated due to standby snapshots.
> <sigh> I can't see why you say this. I am willing to listen, but this > appears to be wrong.
The issue there is that we continue to issue checkpoints if the only activity since the last checkpoint was emitting a standby snapshot. That's because:
I agree this is the current situation in 9.4 and 9.5, hence the bug report.
This no longer occurs with the patches I have proposed. The snapshot is skipped, so a further checkpoint never triggers.
The proposed patch allows to fix that in a more principled manner, because we can simply check that no "important" records have been emitted since the last checkpoint, and skip if that's the case.
I understand the proposal you have made. The patch to implement it is what I object to; my comments made last Sunday.
> What issue is that? Previously you said it must not skip it at all for > logical.
It's fine to skip the records iff nothing important has happened since the last time a snapshot has been logged. Again, the proposed approach allowed to detect that.
Agreed, both proposals do that.
> > We now log more WAL with > > XLogArchiveTimeout > 0 than without.
> And the problem with that is what?
That an idle system unnecessarily produces WAL? Waking up disks and everything?
The OP discussed a problem with archive_timeout > 0. Are you saying we should put in a fix that applies whatever the setting of archive_timeout?
Are we concerned that a master sends a small amount of data every few minutes to a standby it is connected to? I hadn't read that in the thread.
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Simon Riggs http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
