linux-kernel - Re: Ext4 and the "30 second window of death"

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Message-ID: <20090402234751.GC9538@srcf.ucam.org>
Date:	Fri, 3 Apr 2009 00:47:51 +0100
From:	Matthew Garrett <mjg59@...f.ucam.org>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	david@...g.hm, Theodore Tso <tytso@....edu>,
	Sitsofe Wheeler <sitsofe@...oo.com>,
	"Andreas T.Auer" <andreas.t.auer_lkml_73537@...us.ath.cx>,
	Alberto Gonzalez <info@...bu.es>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Ext4 and the "30 second window of death"

On Fri, Apr 03, 2009 at 05:56:40AM +1100, Nick Piggin wrote:
> On Friday 03 April 2009 05:38:34 Matthew Garrett wrote:
> > On Fri, Apr 03, 2009 at 05:34:59AM +1100, Nick Piggin wrote:
> > 
> > > Shouldn't applications have a mode to avoid spinning up the disk if it is
> > > so important?
> > 
> > They do. It's called "Don't use fsync() unless your data needs to be on 
> > disk". I'm not sure why you'd ever want an application to be in anything 
> > but this mode.
> > 
> 
> Well you might decide you are willing to sacrifice timely storage of
> logs, or reducing backups in your editor or something. But obviously
> the kernel can't decide which of those fsyncs is safe to omit (or
> turn into a barrier) while staying within the advertised semantics of
> the app. Application obviously can.

I'd argue that if the user cares enough that they want it fsync()ed on 
ext3 then they probably also want it fsync()ed if they're on battery. 
But yes, if anything is going to make a distinction between grades of 
"Must be saved" then it has to be the application - the kernel certainly 
doesn't have that information.

-- 
Matthew Garrett | mjg59@...f.ucam.org
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