On Fri, May 6, 2011 at 5:31 PM, Greg Smith <greg@2ndquadrant.com> wrote:
> On 05/05/2011 05:06 AM, Mitsuru IWASAKI wrote:
>>
>> In summary, PgFincore's target is File System Buffer Cache, Buffer
>> Cache Hibernation's target is DB Buffer Cache(shared buffers).
>>
>
> Right. The thing to realize is that shared_buffers is becoming a smaller
> fraction of the total RAM used by the database every year. On Windows it's
> been stuck at useful settings being less than 512MB for a while now. And on
> UNIX systems, around 8GB seems to be effective upper limit. Best case,
> shared_buffers is only going to be around 25% of total RAM; worst-case,
> approximately, you might have Windows server with 64GB of RAM where
> shared_buffers is less than 1% of total RAM.
>
> There's nothing wrong with the general idea you're suggesting. It's just
> only targeting a small (and shrinking) subset of the real problem here.
> Rebuilding cache state starts with shared_buffers, but that's not enough of
> the problem to be an effective tweak on many systems.
>
> I think that all the complexity with CRCs etc. is unlikely to lead anywhere
> too, and those two issues are not completely unrelated. The simplest,
> safest thing here is the right way to approach this, not the most
> complicated one, and a simpler format might add some flexibility here to
> reload more cache state too. The bottleneck on reloading the cache state is
> reading everything from disk. Trying to micro-optimize any other part of
> that is moving in the wrong direction to me. I doubt you'll ever measure a
> useful benefit that overcomes the expense of maintaining the code. And you
> seem to be moving to where someone can't restore cache state when they
> change shared_buffers. A simpler implementation might still work in that
> situation; reload until you run out of buffers if shared_buffers shrinks,
> reload until you're done with the original size.
Yeah, I'm pretty well convinced this whole approach is a dead end.
Priming the OS buffer cache seems way more useful. I also think
saving the blocks to be read rather than the actual blocks makes a lot
more sense.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
