O_DIRECT option used?

Hello,

In the FreeBSD 4.5 appear additional O_DIRECT option for open(2),
which mean "eliminate or reduce cache effects". I think, that double
buffering the same disk blocks is ineffective. I grepped all
PostgreSQL sources for this options and didn't find this anywhere.
PgSQL have it's own disk caches, which should be smart
(DB can know what data should be cached first better than OS).
Why wasting OS memory for double caches? OS can use this caches
for other disk blocks, so total cached disk blocks in whole
system will be much bigger. If you have no plans to implement this,
please let us know what source files I should looking for patching.
I think it's somewhere DB tables files are opened (and maybe
WAL files too). Thanks a lot on any comments.

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best regards,
Ruslan A Dautkhanov rusland(at)scn(dot)ru