linux-kernel - Re: New copyfile system call

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Message-ID: <20130330194933.GB1005@amd.pavel.ucw.cz>
Date:	Sat, 30 Mar 2013 20:49:33 +0100
From:	Pavel Machek <pavel@....cz>
To:	Jörn Engel <joern@...fs.org>
Cc:	Andy Lutomirski <luto@...capital.net>, Zach Brown <zab@...hat.com>,
	"Myklebust, Trond" <Trond.Myklebust@...app.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Ric Wheeler <rwheeler@...hat.com>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Chris L. Mason" <clmason@...ionio.com>,
	Christoph Hellwig <hch@...radead.org>,
	Alexander Viro <aviro@...hat.com>,
	"Martin K. Petersen" <mkp@....net>, Hannes Reinecke <hare@...e.de>,
	Joel Becker <jlbec@...lplan.org>
Subject: Re: New copyfile system call - discuss before LSF?

Hi!

> > I thought the first thing people would ask for is to atomically create a
> > new file and copy the old file into it (at least on local file systems).
> >  The idea is that nothing should see an empty destination file, either
> > by race or by crash.  (This feature would perhaps be described as a
> > pony, but it should be implementable.)
> 
> Having already wasted many week trying to implement your pony, I would
> consider it about as possible as winning the lottery three times in a
> row.  It clearly is in theory and yet,...

Hmm, really? AFAICT it would be simple to provide open_deleted_file("directory")
syscall. You'd open_deleted_file(), copy source file into it, then
fsync(), then link it into filesystem.

That should have atomicity properties reflected.
									Pavel
								(who has too many (*)
									ponies around)
(*) 1 is sometimes too many when we talk about big mammals.
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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