Re: Why we are going to have to go DirectIO

On 12/03/2013 10:59 AM, Joshua D. Drake wrote:
> This seems rather half cocked. I read the article. They found a problem,
> that really will only affect a reasonably small percentage of users,
> created a test case, reported it, and a patch was produced.

"Users with at least one file bigger than 50% of RAM" is unlikely to be
a small percentage.

> 
> Kind of like how we do it.

I like to think we'd have at least researched the existing literature on
2Q algorithms (which is extensive) before implementing our own.  Oh,
wait, we *did*.  Nor is this the first ill-considered performance hack
pushed into mainline kernels without any real testing.  It's not even
the first *that year*.

While I am angry over this -- no matter what Kernel.org fixes now, we're
going to have to live with their mistake for 3 years -- the DirectIO
thing isn't just me; when I've gone to Linux Kernel events to talk about
IO, that's the response I've gotten from most Linux hackers: "you
shouldn't be using the filesystem, use DirectIO and implement your own
storage."

That's why they don't feel that it's a problem to break the IO stack;
they really don't believe that anyone who cares about performance should
be using it.

-- 
Josh Berkus
PostgreSQL Experts Inc.
http://pgexperts.com