linux-kernel - Re: New copyfile system call

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Message-ID: <51582294.1000806@draigBrady.com>
Date:	Sun, 31 Mar 2013 12:48:36 +0100
From:	Pádraig Brady <P@...igBrady.com>
To:	Andreas Dilger <adilger@...ger.ca>
CC:	Pavel Machek <pavel@....cz>,
	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 03/30/2013 08:08 PM, Andreas Dilger wrote:
> 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

This reminds me of the flink() discussion:
http://marc.info/?l=linux-kernel&m=104965452917349

Also kinda related is the exchangedata() OSX system call to
"atomically exchange data between two files"

thanks,
Pádraig.
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