Re: a question about Direct I/O and double buffering - Mailing list pgsql-performance

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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