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

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Date:	Thu, 24 Apr 2008 12:29:14 -0400
From:	"Alan D. Brunelle" <Alan.Brunelle@...com>
To:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Cc:	davecb@....com, Jens Axboe <jens.axboe@...cle.com>
Subject: Re: [RFC][PATCH 0/3] Skip I/O merges when disabled

David Collier-Brown wrote:
> 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

No, it's intended for devices being used for /random IO loads/ - like
index seeks during OLTP and the like. But in general, the idea is if you
know you have a highly random IO work load, you can set the tunable and
get back a significant chunk of CPU cycles.

Alan
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