On Mon, May 28, 2012 at 01:34:18PM -0700, Richard Elling wrote:
> I'd be interested in the results of such tests. 

Me too, especially for databases like postgresql where there's a
complementary cache size tunable within the db that often needs to be
turned up, since they implicitly rely on some filesystem caching as a L2. 

That's where this gets tricky: L2ARC has the opportunity to make a big
difference, where the entire db won't all fit in memory (regardless of
which subsystem has jurisdiction over that memory).  If you exclude
data from ARC, you can't spill it to L2ARC.

For the mmap case: does the ARC keep a separate copy, or does the vm
system map the same page into the process's address space?  If a
separate copy is made, that seems like a potential source of many
kinds of problems - if it's the same page then the whole premise is
essentially moot and there's no "double caching".


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