linux-kernel - Re: New copyfile system call

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Date:	Sun, 31 Mar 2013 04:45:22 +0000
From:	"Myklebust, Trond" <Trond.Myklebust@...app.com>
To:	Andy Lutomirski <luto@...capital.net>
CC:	Andreas Dilger <adilger@...ger.ca>,
	Ric Wheeler <rwheeler@...hat.com>, Pavel Machek <pavel@....cz>,
	Jörn Engel <joern@...fs.org>,
	Zach Brown <zab@...hat.com>,
	Paolo Bonzini <pbonzini@...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 Sun, 2013-03-31 at 00:36 -0400, Trond Myklebust wrote:
> On Sat, 2013-03-30 at 21:18 -0700, Andy Lutomirski wrote:
> > On Sat, Mar 30, 2013 at 8:52 PM, Myklebust, Trond
> > <Trond.Myklebust@...app.com> wrote:
> > > On Sat, 2013-03-30 at 19:53 -0700, Andreas Dilger wrote:
> > >> On 2013-03-30, at 16:21, Ric Wheeler <rwheeler@...hat.com> wrote:
> > >>
> > >> > On 03/30/2013 05:57 PM, Myklebust, Trond wrote:
> > >> >> On Mar 30, 2013, at 5:45 PM, Pavel Machek <pavel@....cz>
> > >> >>  wrote:
> > >> >>
> > >> >>> On Sat 2013-03-30 13:08:39, 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.
> > >> >>> Hmm. open_deleted_file() will still need to get a directory... so it
> > >> >>> will still need a path. Perhaps open("/foo/bar/mnt", O_DELETED) would
> > >> >>> be acceptable interface?
> > >> >>>                                    Pavel
> > >> >> ...and what's the big plan to make this work on anything other than ext4 and btrfs?
> > >> >>
> > >> >> Cheers,
> > >> >>   Trond
> > >> >
> > >> > I know that change can be a good thing, but are we really solving a pressing problem given that application developers have dealt with open/rename as the way to get "atomic" file creation for several decades now ?
> > >>
> > >> Using open()+rename() has side effects:
> > >> - changes ctime/mtime on parent directory
> > >> - leaves temporary file in path during creation
> > >> - leaves temporary file in namespace during operations, and after crash
> > >
> > > So what is the actual problem that is being solved? Yes, the above may
> > > be disadvantages, but none of them have proven to be show-stoppers so
> > > far.
> > >
> > > So far, I've seen no justification for Andy's atomicity requirement
> > > other than "it would be nice if...". That's not enough IMO...
> > 
> > ISTM vpsendfile (or whatever it's called) plus a way to create deleted
> > files plus a way to relink deleted files gives atomic copies.  Perhaps
> > this is less efficient than would be ideal for OCFS2, though.
> 
> What real-life problem does the atomicity requirement solve? None of our
> customers have ever asked for it. They don't care...
> 
BTW: before you do answer, please note that the current NFSv4.2 solution
_does_ allow you to lock the file before you copy.

IOW: the same atomicity rules apply to offloaded copy as apply to
standard copy: there is no requirement anywhere to apply stronger
semantics. Surprisingly enough, that works for most people...

-- 
Trond Myklebust
Linux NFS client maintainer

NetApp
Trond.Myklebust@...app.com
www.netapp.com
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