Re: Per table autovacuum vacuum cost limit behaviour strange

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