Message-ID: <54000317.6030604@catalyst.net.nz>
Date: Fri, 29 Aug 2014 16:35:35 +1200
From: Mark Kirkwood <mark.kirkwood@catalyst.net.nz>
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To: Alvaro Herrera <alvherre@2ndquadrant.com>,
 Robert Haas <robertmhaas@gmail.com>
CC: Amit Kapila <amit.kapila16@gmail.com>,
 Haribabu Kommi <kommi.haribabu@gmail.com>,
 "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Subject: Re: Per table autovacuum vacuum cost limit behaviour strange
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On 29/08/14 08:56, Alvaro Herrera wrote:
> Robert Haas wrote:
>
>> I agree that you might not like that.  But you might not like having
>> the table vacuumed slower than the configured rate, either.  My
>> impression is that the time between vacuums isn't really all that
>> negotiable for some people.  I had one customer who had horrible bloat
>> issues on a table that was vacuumed every minute; when we changed the
>> configuration so that it was vacuumed every 15 seconds, those problems
>> went away.
>
> Wow, that's extreme.  For that case you can set
> autovacuum_vacuum_cost_limit to 0, which disables the whole thing and
> lets vacuum run at full speed -- no throttling at all.  Would that
> satisfy the concern?
>

Well no - you might have a whole lot of big tables that you want vacuum 
to not get too aggressive on, but a few small tables that are highly 
volatile. So you want *them* vacuumed really fast to prevent them 
becoming huge tables with only a few rows therein, but your system might 
not be able to handle *all* your tables being vacuum full speed.

This is a fairly common scenario for (several) web CMS systems that tend 
to want to have session and/cache tables that are small and extremely 
volatile, plus the rest of the (real) data that is bigger and vastly 
less volatile.

While there is a valid objection along the lines of "don't use a 
database instead of memcache", it does seem reasonable that Postgres 
should be able to cope with this type of workload.

Cheers

Mark


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