Re: O_DIRECT in freebsd

Bruce Momjian <pgman@candle.pha.pa.us> writes:
> True, it is a cost/benefit issue.  My assumption was that once we have
> free-behind in the PostgreSQL shared buffer cache, the kernel cache
> issues would be minimal, but I am willing to be found wrong.

If you are running on the small-shared-buffers-and-large-kernel-cache
theory, then getting the kernel cache to behave right is much more
important than making the PG cache behave right.  If you favor the
other theory then free-behind in the PG cache is the important thing.
However, I've not yet seen any convincing evidence that large PG
cache with small kernel cache is the way to go.

The scenario that I'm envisioning would require passing a this-is-a-
seqscan-read flag down through bufmgr to smgr to fd.c.  So the info
would be available to bufmgr to implement a free-behind policy.  But
I suspect that's not really where any big win would come from.
        regards, tom lane