Re: About to add WAL write/fsync statistics to pg_stat_wal view

On 2021/03/23 16:10, Fujii Masao wrote:
>
>
> On 2021/03/22 20:25, ikedamsh wrote:
>> Agreed. Users can know whether the stats is for walreceiver or not. The
>> pg_stat_wal view in standby server shows for the walreceiver, and in primary
>> server it shows for the others. So, I updated the document.
>> (v20-0003-Makes-the-wal-receiver-report-WAL-statistics.patch)
>
> Thanks for updating the docs!
>
> There was the discussion about when the stats collector is invoked, at [1].
> Currently during archive recovery or standby, the stats collector is
> invoked when the startup process reaches the consistent state, sends
> PMSIGNAL_BEGIN_HOT_STANDBY, and then the system is starting accepting
> read-only connections. But walreceiver can be invoked at earlier stage.
> This can cause walreceiver to generate and send the statistics about WAL
> writing even though the stats collector has not been running yet. This might
> be problematic? If so, maybe we need to ensure that the stats collector is
> invoked before walreceiver?
>
> During recovery, the stats collector is not invoked if hot standby mode is
> disabled. But walreceiver can be running in this case. So probably we should
> change walreceiver so that it's invoked even when hot standby is disabled?
> Otherwise we cannnot collect the statistics about WAL writing by walreceiver
> in that case.
>
> [1]
> https://postgr.es/m/e5a982a5-8bb4-5a10-cf9a-40dd1921bdb5@oss.nttdata.com

Thanks for comments! I didn't notice that.
As I mentioned[1], if my understanding is right, this issue seem to be not for
only the wal receiver.

Since the shared memory thread already handles these issues, does this patch,
which to collect the stats for the wal receiver and make a common function for
writing wal files, have to be committed after the patches for share memory
stats are committed? Or to handle them in this thread because we don't know
when the shared memory stats patches will be committed.

I think the former is better because to collect stats in shared memory is very
useful feature for users and it make a big change in design. So, I think it's
beneficial to make an effort to move the shared memory stats thread forward
(by reviewing or testing) instead of handling the issues in this thread.

[1]
https://www.postgresql.org/message-id/9f4e19ad-518d-b91a-e500-25a666471c42%40oss.nttdata.com

Regards,
--
Masahiro Ikeda
NTT DATA CORPORATION