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From: Dave Cramer <pg@fastcrypt.com>
Subject: Re: Vacuums on large busy databases
Date: Thu, 14 Sep 2006 20:07:09 -0400
To: Francisco Reyes <lists@stringsutils.com>


On 14-Sep-06, at 7:50 PM, Francisco Reyes wrote:

> Dave Cramer writes:
>
>> personally, I'd set this to about 6G. This doesn't actually  
>> consume  memory it is just a setting to tell postgresql how much  
>> memory is  being used for cache and kernel buffers
>
> Gotcha. Will increase further.
>
>> regarding shared buffers I'd make this much bigger, like 2GB or more
>
> Will do 2GB on the weekend. From what I read this requires shared  
> memory so have to restart my machine (FreeBSD).
>
> if I plan to give shared buffers 2GB, how much more over that  
> should I give the total shared memory kern.ipc.shmmax? 2.5GB?

I generally make it slightly bigger. is shmmax the size of the  
maximum chunk allowed or the total ?
>
> Also will shared buffers impact inserts/updates at all?
> I wish the postgresql.org site docs would mention what will be  
> impacted.
Yes, it will, however not as dramatically as what you are seeing with  
effective_cache
>
> Comments like: This setting must be at least 16, as well as at  
> least twice the value of max_connections; however, settings  
> significantly higher than the minimum are usually needed for good  
> performance.
>
> Are usefull, but could use some improvement.. increase on what? All  
> performance? inserts? updates? selects?
>
> For instance, increasing effective_cache_size has made a noticeable  
> difference in selects. However as I talk to the developers we are  
> still doing marginally in the inserts. About 150/min.
The reason is that with effective_cache the select plans changed (for  
the better) ; it's unlikely that the insert plans will change.
>
> There is spare CPU cycles, both raid cards are doing considerably  
> less they can do.. so next I am going to try and research what  
> parameters I need to bump to increase inserts. Today I increased  
> checkpoint_segments from the default to 64. Now looking at  
> wall_buffers.
>
> It would be most helpfull to have something on the docs to specify  
> what each setting affects most such as reads, writes, updates,  
> inserts, etc..
It's an art unfortunately.
>

Dave