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

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

Regards,

--
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION