On Tue, Jan 14, 2014 at 5:08 AM, Hannu Krosing <hannu@2ndquadrant.com> wrote:
> Again, as said above the linux file system is doing fine. What we
> want is a few ways to interact with it to let it do even better when
> working with postgresql by telling it some stuff it otherwise would
> have to second guess and by sometimes giving it back some cache
> pages which were copied away for potential modifying but ended
> up clean in the end.
You don't need new interfaces. Only a slight modification of what
fadvise DONTNEED does.
This insistence in injecting pages from postgres to kernel is just a
bad idea. At the very least, it still needs postgres to know too much
of the filesystem (block layout) to properly work. Ie: pg must be
required to put entire filesystem-level blocks into the page cache,
since that's how the page cache works. At the very worst, it may
introduce serious security and reliability implications, when
applications can destroy the consistency of the page cache (even if
full access rights are checked, there's still the possibility this
inconsistency might be exploitable).
Simply making fadvise DONTNEED move pages to the head of the LRU (ie:
discard next if you need) should work as expected without all the
complication of the above proposal.