linux-kernel - Re: [rfc] Ignore Fsync Calls in Laptop

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Date:	Mon, 23 May 2011 10:22:02 +0200 (CEST)
From:	Jesper Juhl <jj@...osbits.net>
To:	"D. Jansen" <d.g.jansen@...glemail.com>
cc:	Valdis.Kletnieks@...edu, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, tytso@....edu
Subject: Re: [rfc] Ignore Fsync Calls in Laptop_Mode

On Fri, 20 May 2011, D. Jansen wrote:

> On Fri, May 20, 2011 at 5:28 PM,  <Valdis.Kletnieks@...edu> wrote:
> > On Thu, 19 May 2011 15:34:46 +0200, Dennis Jansen said:
> >
> >> Testing:
> >> I've been using this workaround on my netbook for over six months now.
> >> It works as expected for me with all software in a Ubuntu 9.10
> >> environment and saves me at least 0.5 Watt or roughly 10 % battery
> >> time - without destroying my hard disk. I have seen no negative side
> >> effects.
> >
> > How much destructive testing did you do?  In the 6 months, how many times did
> > the system crash (or had the battery pulled out, or whatever) while large
> > amounts of data were still pending after apps thought they were fsync'ed? How
> > much crash testing was done against apps that use fsync for ordering or
> > correctness reasons?
> 
> I don't see the point in verifying the obvious. Of course applications
> that rely on fsync will lose data.
> The real problem comes with ordering correctness, which could actually
> _destroy previous data_ as well.
> In my scenario (office applications, browsing) I have not hit such a problem.
> 
> Does anyone know a Linux app that actually does rely on ordering
> correctness? Is that one which is used on a laptop? In laptop mode?
> (-> when on battery?) Because so far the discussion seems to be
> running in circles around Alan's mailer daemon. I would just shut that
> down on enabling laptop mode. Problem solved. But I don't run that on
> my laptop, anyway, esp. in laptop mode. I wonder who would, too.
> 

PostgreSQL's Write Ahead Log (WAL) would be one such example as far as I 
can see ( http://www.westnet.com/~gsmith/content/postgresql/TuningPGWAL.htm ).
And yes, people run that on laptops while on battery (I know that for a 
fact since I do that myself).

-- 
Jesper Juhl <jj@...osbits.net>       http://www.chaosbits.net/
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