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

[David Collier-Brown <davecb@sun.com>]

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
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