On Mon, Sep 9, 2013 at 6:29 PM, Bruce Momjian <bruce@momjian.us> wrote:
> On Thu, Sep 5, 2013 at 09:02:27PM -0700, Josh Berkus wrote:
>> On 09/05/2013 03:30 PM, Merlin Moncure wrote:
>>
>> >> Standard advice we've given in the past is 25% shared buffers, 75%
>> >> effective_cache_size. Which would make EFS *3X* shared_buffers, not 4X.
>> >> Maybe we're changing the conventional calculation, but I thought I'd
>> >> point that out.
>> >
>> > This was debated upthread.
>>
>> Actually, no, it wasn't. Tom threw out a suggestion that we use 4X for
>> historical reasons. That's all, there was no discussion.
>>
>> So, my point stands: our historical advice has been to set EFS to 75% of
>> RAM. Maybe we're changing that advice, but if so, let's change it.
>> Otherwise 3X makes more sense.
>
> So, what do we want the effective_cache_size default to be? 3x or 4x?
> We clearly state:
>
> If you have a dedicated database server with 1GB or more of RAM,
> a reasonable starting value for shared_buffers is 25% of the
> memory in your system. There are some workloads where even
>
> If we make the default 4x, that means that people using the above
> suggestion would be setting their effective_cache_size to 100% of RAM?
> If we go with 4x, which I believe was the majority opinion, what shall
> we answer to someone who asks about this contradiction?
I vote for 3x. The past defaults had a different ratio, but we are
changing things to make them better, not to leave them the same. We
should change it be consistent with the advice we have long given.
Sure, 3 is not a power of 2, but I usually root for the underdog.
Cheers,
Jeff
