What is it about the buffer cache that makes it so unhappy being able to
hold everything? I don't want to be seen as a cache hit fascist, but isn't
it just better if the data is just *there*, available in the postmaster's
address space ready for each backend process to access it, rather than
expecting the Linux cache mechanism, optimised as it may be, to have to do
the caching?

Because the PostgreSQL buffer management algorithms are pitiful compared to Linux's. In 7.5, it's improved with the new ARC algorithm, but still - the Linux disk buffer cache will be very fast.


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