linux-kernel - Re: New copyfile system call

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Message-ID: <20130311093134.GI7783@localhost>
Date:	Mon, 11 Mar 2013 02:31:35 -0700
From:	Joel Becker <jlbec@...lplan.org>
To:	Zach Brown <zab@...hat.com>
Cc:	"Myklebust, Trond" <Trond.Myklebust@...app.com>,
	Andy Lutomirski <luto@...capital.net>,
	Ric Wheeler <rwheeler@...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>
Subject: Re: New copyfile system call - discuss before LSF?

On Mon, Feb 25, 2013 at 04:03:01PM -0800, Zach Brown wrote:
> > >   I think it would be neat if it couldn't
> > > corrupt data.
> > 
> > It would also be neat if the moon were made of cheese...
> 
> And there we have the lsf2013 t-shirt slogan.  I think we're done here!
> 
> - z

Hey Everyone,
	So, of course, this thread happened while I was celebrating my
10-year anniversary on a warm, sunny island.  I won't trade.  But let me
drop my $0.02 in here.
	First, we have our T-shirt slogan.  That overrides every other
concern.
	Second, I agree that moving forward on anything is better than
not.  I haven't delivered the updated fastcopy(2) patch I promised two
years ago, and I have to admit that I can't promise code on any sane
timeframe.
	Back when I was working on this, I thought that link(2) was a
good model for a full-file copy.  Thus I came up with reflink(2).  This
eventually became the fastcopyu(2) proposal discussed two years ago.  I
did not think, and I still don't think, that we should conflate the API
for "copy/clone this file in some way" (ala fastcopy(2)) with
"duplicate/link this range of bytes" (ala BTRFS_IOC_CLONE_RANGE).  I
thought that splice(2) or something like it was a better fit for ranges;
this thread has already had the same thought.
	fastcopy(2) had a provision for CoW for atomicity, including
metadata.  This is because ocfs2 reflinks *can* provide atomic clones
with metadata included.  I would like any new proposal to allow for
that.  If it does not, of course, callers can continue to use
OCFS2_IOC_REFLINK, but I'd rather make it part of the generic behavior,
so that generic tools come with it.

Joel

-- 

"You don't make the poor richer by making the rich poorer."
	- Sir Winston Churchill

			http://www.jlbec.org/
			jlbec@...lplan.org
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