On Tue, Jan 14, 2014 at 4:23 PM, James Bottomley
<james.bottom...@hansenpartnership.com> wrote:
> Yes, that's what I was thinking: it's a cache.  About how many files
> comprise this cache?  Are you thinking it's too difficult for every
> process to map the files?

No, I'm thinking that would throw cache coherency out the window.
Separate mappings are all well and good until somebody decides to
modify the page, but after that point the database processes need to
see the modified version of the page (which is, further, hedged about
with locks) yet the operating system MUST NOT see the modified version
of the page until the write-ahead log entry for the page modification
has been flushed to disk.  There's really no way to do that without
having our own private cache.

Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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