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

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Date:	Thu, 24 Apr 2008 11:53:14 -0400
From:	David Collier-Brown <davecb@....com>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	"Alan D. Brunelle" <Alan.Brunelle@...com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH 0/3] Skip I/O merges when disabled

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@....com                 |                      -- Mark Twain
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