Re: a question about Direct I/O and double buffering

From: Mark Lewis
Subject: Re: a question about Direct I/O and double buffering
Date: ,
Msg-id: 1175802972.5581.56.camel@archimedes
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In response to: Re: a question about Direct I/O and double buffering  (Erik Jones)
Responses: Re: a question about Direct I/O and double buffering  (Erik Jones)
List: pgsql-performance

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a question about Direct I/O and double buffering  (Xiaoning Ding <-state.edu>, )
 Re: a question about Direct I/O and double buffering  (Erik Jones, )
  Re: a question about Direct I/O and double buffering  (Xiaoning Ding <-state.edu>, )
   Re: a question about Direct I/O and double buffering  (Erik Jones, )
    Re: a question about Direct I/O and double buffering  ("Alex Deucher", )
     Re: a question about Direct I/O and double buffering  (Xiaoning Ding <-state.edu>, )
      Re: a question about Direct I/O and double buffering  ("Alex Deucher", )
      Re: a question about Direct I/O and double buffering  (, )
   Re: a question about Direct I/O and double buffering  (, )
    Re: a question about Direct I/O and double buffering  (Erik Jones, )
     Re: a question about Direct I/O and double buffering  (, )
  Re: a question about Direct I/O and double buffering  (Mark Lewis, )
   Re: a question about Direct I/O and double buffering  (Erik Jones, )
    Re: a question about Direct I/O and double buffering  (Mark Lewis, )
     Re: a question about Direct I/O and double buffering  (Erik Jones, )
      Re: a question about Direct I/O and double buffering  ("Jim C. Nasby", )

...
[snipped for brevity]
...
>
> > Not to hijack this thread, but has anybody here tested the behavior
> > of
> > PG on a file system with OS-level caching disabled via forcedirectio
> > or
> > by using an inherently non-caching file system such as ocfs2?
> >
> >
> > I've been thinking about trying this setup to avoid double-caching
> > now
> > that the 8.x series scales shared buffers better, but I figured I'd
> > ask
> > first if anybody here had experience with similar configurations.
> >
> >
> > -- Mark
>
>
> Rather than repeat everything that was said just last week, I'll point
> out that we just had a pretty decent discusson on this last week that
> I started, so check the archives.  In summary though, if you have a
> high io transaction load with a db where the average size of your
> "working set" of data doesn't fit in memory with room to spare, then
> direct io can be a huge plus, otherwise you probably won't see much of
> a difference.  I have yet to hear of anybody actually seeing any
> degradation in the db performance from it.  In addition, while it
> doesn't bother me, I'd watch the top posting as some people get pretty
> religious about (I moved your comments down).

I saw the thread, but my understanding from reading through it was that
you never fully tracked down the cause of the factor of 10 write volume
mismatch, so I pretty much wrote it off as a data point for
forcedirectio because of the unknowns.  Did you ever figure out the
cause of that?

-- Mark Lewis


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