Re: patch for new feature: Buffer Cache Hibernation

2011/5/15 Robert Haas <robertmhaas(at)gmail(dot)com>:
> On Fri, May 6, 2011 at 5:31 PM, Greg Smith <greg(at)2ndquadrant(dot)com> wrote:
>> 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.
>
> I don't think there's any need for this to get data into
> shared_buffers at all.  Getting it into the OS cache oughta be plenty
> sufficient, no?
>
> ISTM that a very simple approach here would be to save the contents of
> each shared buffer on clean shutdown, and to POSIX_FADV_WILLNEED those
> buffers on startup.

+1
It is just an evolution of the current process if I understood the
explantions of the latest patch correctly.

>We could worry about additional complexity, like
> using fincore to probe the OS cache, in a follow-on patch.  While
> reloading only 8GB of maybe 30GB of cached data on restart would not
> be as good as reloading all of it, it would be a lot better than
> reloading none of it, and the gymnastics required seems substantially
> less.
>
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> Robert Haas
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Cédric Villemain               2ndQuadrant
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