linux-kernel - Re: [PATCH 0/8][RFC] IO latency/throughput fixes

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Message-ID: <alpine.LFD.2.00.0904061509020.7443@localhost.localdomain>
Date:	Mon, 6 Apr 2009 15:17:28 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ray Lee <ray-lk@...rabbit.org>
cc:	Hua Zhong <hzhong@...il.com>, Theodore Tso <tytso@....edu>,
	Jens Axboe <jens.axboe@...cle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/8][RFC] IO latency/throughput fixes

On Mon, 6 Apr 2009, Ray Lee wrote:
> 
> Your argument seems to be that someone may be relying upon default
> kernel behavior and, at the same time, is willing to continually
> upgrade their kernel.

No, I do think that the argument is valid per se: the crash behavior of 
"data=ordered" wrt "data=writeback" _is_ very different. And it's true 
that "data=ordered" has nicer behavior in the case of a crash.

But it is also true that with all the recent patches, "data=writeback" has 
become a _lot_ more attractive. It is slightly more ordered than it used 
to be, in a very particular way that makes one of the biggest downsides go 
away. And the latency improvements are really quite stunning (*). 

So I think Hua's argument is real, but at the same time we should balance 
it against the fact that a lot of people will just use whatever is the 
default - and as a result we should strive to make the default be the 
thing that we think people would be happiest with.

I think "ordered" was a reasonable default, but that was at least partly 
because _both_ ordered and writeback sucked (partly in different ways).

I do think we could make it a config option.

			Linus

(*) Admittedly with a very specific workload, but I suspect it's not that 
uncommon, and there will be people who never realized what their problem 
was and blamed jerky behavior on scheduler or something.
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