Re: how to plan for vacuum?

Jim C. Nasby wrote:
> On Wed, Jan 24, 2007 at 02:37:44PM +0900, Galy Lee wrote:
>> 1.  How do we know if autovacuum is enough for my application, or should
>>     I setup a vacuum manually from cron for my application?
>
> Generally I trust autovac unless there's some tables where it's critical
> that they be vacuumed frequently, such as a queue table or a web session
> table.

So how much can we trust autovac? I think at least the following cases
can not be covered by autovac now:
  - small but high update tables which are sensitive to garbage
  - very big tables which need a long time to be vacuumed.
  - when we need to adjust the the max_fsm_page

>> 2. How to set the GUC parameters for autovacuum?
>> There are two sets of parameters for autovacuum:
>>     - vacuum threshold and scale factor (500/0.2)
>> ?$B!!  - analyze threshold and scale factor(250/0.1)
>> Is there any guideline to set these parameters?  When does it need to
>> change the default values?
>
> I find those are generally pretty good starting points; just bear in
> mind that it means 20% dead space.

so what is the principle to set them?
  - keep dead space lower than some disk limit
  - or keep the garbage rate lower than fillfactor
  or any other general principle?