Re: [RFC] Should we fix postmaster to avoid slow shutdown?

From: Andres Freund <andres(at)anarazel(dot)de>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, "Tsunakawa, Takayuki" <tsunakawa(dot)takay(at)jp(dot)fujitsu(dot)com>, Peter Eisentraut <peter(dot)eisentraut(at)2ndquadrant(dot)com>, Ashutosh Bapat <ashutosh(dot)bapat(at)enterprisedb(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: [RFC] Should we fix postmaster to avoid slow shutdown?
Date: 2016-11-22 20:55:13
Message-ID: 20161122205512.6tqvebfgpd6uta2i@alap3.anarazel.de
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Hi,

On 2016-11-22 15:49:27 -0500, Robert Haas wrote:
> I think you are almost right. When the server is running, there are
> files in pg_stat_tmp but not pg_stat; when it is shut down, there are
> files in pg_stat but not pg_stat_tmp. Of course the data can never be
> ONLY in the collector's backend-local memory because then nobody else
> could read it.

> I don't actually really understand the reason for this distinction.

pg_stat_tmp commonly is placed on tmpfs/a ramdisk.

But I'm a bit confused too - does this make any sort of difference?
Because the startup path for crash recovery is like this:

void
StartupXLOG(void)
{
...
/* REDO */
if (InRecovery)
{
...
/*
* Reset pgstat data, because it may be invalid after recovery.
*/
pgstat_reset_all();

so it seems quite inconsequential whether we write out pgstat, because
we're going to nuke it either way after an immediate shutdown?

Greetings,

Andres Freund

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