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Re: RFC: Timing Events

From: Josh Berkus <josh(at)agliodbs(dot)com>
To: pgsql-hackers(at)postgresql(dot)org
Subject: Re: RFC: Timing Events
Date: 2012-11-03 01:44:25
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Lists: pgsql-hackers
> I don't see all that going into core without a much bigger push than I
> think people will buy.  What people really want for all these is a
> proper trending system, and that means graphs and dashboards and
> bling--not a history table.

Well, I'm particularly thinking for autoconfiguration.  For example, to
set vacuum_freeze_min_age properly, you have to know the XID "burn rate"
of the server, which is only available via history.  I really don't want
to be depending on a graphical monitoring utility to find these things out.

> This whole approach has the assumption that things are going to fall off
> sometimes.  To expand on that theme for a second, right now I'm more
> worried about the "99%" class of problems.  Neither pg_stat_statements
> nor this idea are very good for tracking the rare rogue problem down.
> They're both aimed to make things that happen a lot more statistically
> likely to be seen, by giving an easier UI to glare at them frequently.
> That's not ideal, but I suspect really fleshing the whole queue consumer
> -> table idea needs to happen to do much better.

I'm just concerned that for some types of incidents, it would be much
more than 1% *of what you want to look at* which fall off.  For example,
consider a server which does 95% reads at a very high rate, but has 2%
of its writes cronically having lock waits.  That's something you want
to solve, but it seems fairly probably that these relatively infrequent
queries would have fallen off the bottom of pg_stat_statements.  Same
thing with the relative handful of queries which do large on-disk sorts.

The problem I'm worried about is that pg_stat_statements is designed to
keep the most frequent queries, but sometimes the thing you really need
to look at is not in the list of most frequent queries.

Josh Berkus
PostgreSQL Experts Inc.

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