linux-ext4 - Re: Zero length files - an alternative approach?

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Message-ID: <20090329134901.GB13737@elf.ucw.cz>
Date:	Sun, 29 Mar 2009 15:49:01 +0200
From:	Pavel Machek <pavel@....cz>
To:	M?ns Rullg?rd <mans@...sr.com>
Cc:	"Andreas T.Auer" <andreas.t.auer_lkml_73537@...us.ath.cx>,
	linux-kernel@...r.kernel.org, linux-ext4@...r.kernel.org
Subject: Re: Zero length files - an alternative approach?

On Sun 2009-03-29 13:10:23, M?ns Rullg?rd wrote:
> "Andreas T.Auer" <andreas.t.auer_lkml_73537@...us.ath.cx> writes:
> 
> > On 29.03.2009 13:22 M?ns Rullg?rd wrote:
> >> Consider this scenario:
> >>
> >> 1. Create/write/close newfile
> >> 2. Rename newfile to oldfile
> >> 3. Open/read oldfile.  This must return the new contents.
> >> 4. System crash and reboot before delayed allocation/flush complete
> >> 5. Open/read oldfile.  Old contents now returned.
> >>
> >> This rollback isn't obviously, to me at least, without problems of its
> >> own.
> >>   
> > Having the old data in 5) is far better than having no data in 5).
> 
> Of course having old data is better than no data.  However, fsync()
> and similar approaches make a rollback to old data after new data has
> been visible impossible or far less likely than the suggested one.

Untrue. Unless you fsync after rename, you can get olddata.

fsync() is easy. But some people _want_ to have either newdata _or_
olddata, but don't care which one, and would prefer to avoid
fsync. That's where replace() should help...
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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