Re: O_DIRECT setting

TODO has:

    * Consider use of open/fcntl(O_DIRECT) to minimize OS caching

Should the item be removed?

---------------------------------------------------------------------------

Neil Conway wrote:
> On Mon, 2004-09-20 at 17:57, Guy Thornley wrote:
> > According to the manpage, O_DIRECT implies O_SYNC:
> >
> >         File I/O is done directly to/from user space buffers.  The I/O is
> >         synchronous, i.e., at the completion of the read(2) or write(2)
> >         system call, data is guaranteed to have been transferred.
>
> This seems like it would be a rather large net loss. PostgreSQL already
> structures writes so that the writes we need to hit disk immediately
> (WAL records) are fsync()'ed -- the kernel is given more freedom to
> schedule how other writes are flushed from the cache. Also, my
> recollection is that O_DIRECT also disables readahead -- if that's
> correct, that's not what we want either.
>
> BTW, using O_DIRECT has been discussed a few times in the past. Have you
> checked the list archives? (for both -performance and -hackers)
>
> > Would people be interested in a performance benchmark?
>
> Sure -- I'd definitely be curious, although as I said I'm skeptical it's
> a win.
>
> > I need some benchmark tips :)
>
> Some people have noted that it can be difficult to use contrib/pgbench
> to get reproducible results -- you might want to look at Jan's TPC-W
> implementation or the OSDL database benchmarks:
>
> http://pgfoundry.org/projects/tpc-w-php/
> http://www.osdl.org/lab_activities/kernel_testing/osdl_database_test_suite/
>
> > Incidentally, postgres heap files suffer really, really bad fragmentation,
> > which affects sequential scan operations (VACUUM, ANALYZE, REINDEX ...)
> > quite drastically. We have in-house patches that somewhat alleiviate this,
> > but they are not release quality.
>
> Can you elaborate on these "in-house patches"?
>
> -Neil
>
>
>
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