linux-kernel - Re: [RFC PATCH] fpathconf() for fsync() behavior

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Message-ID: <20090426092601.GC1354@ucw.cz>
Date:	Sun, 26 Apr 2009 11:26:01 +0200
From:	Pavel Machek <pavel@....cz>
To:	Valerie Aurora Henson <vaurora@...hat.com>
Cc:	Theodore Tso <tytso@....edu>, Jamie Lokier <jamie@...reable.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Chris Mason <chris.mason@...cle.com>,
	Eric Sandeen <sandeen@...hat.com>,
	Ric Wheeler <rwheeler@...hat.com>
Subject: Re: [RFC PATCH] fpathconf() for fsync() behavior

Hi!

> > Application writers don't care about OS portability (it only has to
> > work on Linux), or working on multiple filesystems (it only has work
> > on ext3, and any filesystems which doesn't do automagic fsync's at the
> > right magic times automagically is broken by design).  This includes
> > many GNOME and KDE developers.  So as we concluded at the filesystem
> > and storage workshop, we probably will have to keep automagic
> > hueristics out there, for all of the broken applications.  Heck, Linus
> > even refused to call those applications "broken".  
> > 
> > So we can create a more finer-grained controlled system call ---
> > although I would suggest that we just add some extra flags to
> > sync_file_range() --- but it's doubtful that many application
> > programmers will use it.
> 
> I remain hopeful. :) Application developers *want* to do the right
> thing in general; they are just facing a hopeless catch-22 right now.
> The POSIX-ly correct use of fsync() exposes them to potential
> multi-second delays on 95% of file systems currently in existence -
> and the fsync() isn't even needed in many cases!

Perhaps we should create documentation file explaining what kernel
guarantees and what works mostly by mistake?

For example (and I tried to watch the discussion...):

On ext3 data=ordered, is "echo new > bar.new; mv -f bar.new bar "
actually safe?

Is it still safe on ext4 with workarounds, or is it that you just
shrank the window?
								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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