Re: O_DIRECT in freebsd

Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> writes:
> Basically, I think we need free-behind rather than O_DIRECT.

There are two separate issues here --- one is what's happening in our
own cache, and one is what's happening in the kernel disk cache.
Implementing our own free-behind code would help in our own cache but
does nothing for the kernel cache.

My thought on this is that for large seqscans we could think about
doing reads through a file descriptor that's opened with O_DIRECT.
But writes should never go through O_DIRECT. In some scenarios this
would mean having two FDs open for the same relation file. This'd
require moderately extensive changes to the smgr-related APIs, but
it doesn't seem totally out of the question. I'd kinda like to see
some experimental evidence that it's worth doing though. Anyone
care to make a quick-hack prototype and do some measurements?

regards, tom lane