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.
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