Re: Function to know last log write timestamp

On 2014-08-11 12:42:06 +0900, Fujii Masao wrote:
> On Mon, Aug 11, 2014 at 10:48 AM, Tatsuo Ishii <ishii(at)postgresql(dot)org> wrote:
> >> On Mon, Aug 11, 2014 at 9:23 AM, Tatsuo Ishii <ishii(at)postgresql(dot)org> wrote:
> >>> We can know the LSN of last committed WAL record on primary by using
> >>> pg_current_xlog_location(). It seems there's no API to know the time
> >>> when the WAL record was created. I would like to know standby delay by
> >>> using pg_last_xact_replay_timestamp() and such that API.
> >>>
> >>> If there's no such a API, it would be useful to invent usch an API IMO.
> >>
> >> +1
> >>
> >> I proposed that function before, but unfortunately it failed to be applied.
> >> But I still think that function is useful to calculate the replication delay.
> >> The past discussion is
> >> http://www.postgresql.org/message-id/CAHGQGwF3ZjfuNEj5ka683KU5rQUBtSWtqFq7g1X0g34o+JXWBw@mail.gmail.com
> >
> > I looked into the thread briefly and found Simon and Robert gave -1
> > for this because of performance concern. I'm not sure if it's a actual
> > performance penalty or not. Maybe we need to major the penalty?
>
> I think that the performance penalty is negligible small because the patch
> I posted before added only three stores to shared memory per
> commit/abort.

Uh. It adds another atomic operation (the spinlock) to the commit
path. That's surely *not* insignificant. At the very least the
concurrency approach needs to be rethought.

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
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