Re: Tuning Effective Cache Question

Sorry, forgot to state that we are still on PG 7.3.4.

On 8/17/05, Chris Hoover <revoohc(at)gmail(dot)com> wrote:
> I have some questions about tuning my effective_cache_size
>
> I have a RHEL 2.1 box running with dual Xeon (2.6 GHz I believe and
> they have HT on). The box has 8GB memory. In my postgresql.conf, I
> have set the effective_cache_size = 530505 (~4GB).
>
> However, I am noticing on this machine, top is telling me that I have
> ~3.5GB in the buff and only 3GB in cached.
>
> Here are the exact numbers:
> Mem: 7720040K av, 7714364K used, 5676K free, 314816K shrd, 3737540K buff
> Swap: 2096440K av, 119448K used, 1976992K free 3188192K cached
>
>
> 1. Is the configuration that linux is running in hurting PostgreSQL in any way?
>
> 2. Is there a negative impact of the effective_cache_size being
> larger than the actual cached memory?
>
> 3. What effect postive or negative does the buff memory have on PostgreSQL.\
>
> 4. What exactly is this buff memory used for? We have had a time
> trying to find a good explanation of what it means.
>
> Overall, this system appears to be running fine. However, I was taken
> back when I saw the current memory configuration. We have been
> walking a fine line performance wise, and can quickly become i/o
> starved. So I want to make sure that I am not pushing the db towards
> the i/o starved side.
>
> Thanks for any insight,
>
> Chris
>