Re: [RFC][PATCH 0/3] Skip I/O merges when disabled

[David Collier-Brown]

Jens Axboe wrote:
> On Thu, Apr 24 2008, Andi Kleen wrote:
>
> > > Not a good idea IMHO, it's much better with an explicit setting. That
> > > way you don't introduce indeterministic behavior.
> >
> > So you would be deterministically slower.
>
>
> Yes, absolutely. Think about the case for a second - the potential gain is in
> fractions of a percent basically, the potential loss however is HUGE.
> There's absolutely no way on earth I'd ever make this dynamic.

 If this is intended for databases, it might be backwards (;-)) 

 The commercial unix "forcedirectio" option that Oracle and other
database vendors usually ask for turns out to be a benefit
in large sequential data transfers, because it does two things:

1) transfers directly between user address space and disk, avoiding buffering, and
2) allows enthusiastic coalescence of synchronous writes

 Is this intended for DBMSs, or for something esle?

--dave 
--
David Collier-Brown            | Always do right. This will gratify
Sun Microsystems, Toronto      | some people and astonish the rest
davecb@xxxxxxx                 |                      -- Mark Twain
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