linux-kernel - Re: New copyfile system call

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Date:	Sat, 30 Mar 2013 13:08:39 -0700
From:	Andreas Dilger <adilger@...ger.ca>
To:	Pavel Machek <pavel@....cz>
Cc:	Jörn Engel <joern@...fs.org>,
	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?

On 2013-03-30, at 12:49 PM, Pavel Machek wrote:
> Hmm, really? AFAICT it would be simple to provide an
> 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.

Actually, the open_deleted_file() syscall is quite useful for many
different things all by itself.  Lots of applications need to create
temporary files that are unlinked at application failure (without a
race if app crashes after creating the file, but before unlinking).
It also avoids exposing temporary files into the namespace if other
applications are accessing the directory.

We've added a library routine that does this for Lustre in a hackish
way (magical filename created in target directory) for being able to
migrate files between data servers, HSM, defragmentation, rsync, etc.

Cheers, Andreas

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