Date: Sun, 06 Oct 2013 03:42:35 -0500
From: Rob Landley <rob@landley.net>
Subject: Re: [RFC] extending splice for copy offloading
To: Miklos Szeredi <miklos@szeredi.hu>
Cc: "J. Bruce Fields" <bfields@fieldses.org>, Zach Brown <zab@redhat.com>,
        Anna Schumaker <schumaker.anna@gmail.com>,
        Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux-Fsdevel <linux-fsdevel@vger.kernel.org>,
        "linux-nfs@vger.kernel.org" <linux-nfs@vger.kernel.org>,
        Trond Myklebust <Trond.Myklebust@netapp.com>,
        Bryan Schumaker <bjschuma@netapp.com>,
        "Martin K. Petersen" <mkp@mkp.net>, Jens Axboe <axboe@kernel.dk>,
        Mark Fasheh <mfasheh@suse.com>, Joel Becker <jlbec@evilplan.org>,
        Eric Wong <normalperson@yhbt.net>
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	(from miklos@szeredi.hu on Thu Sep 26 13:06:41 2013)
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On 09/26/2013 01:06:41 PM, Miklos Szeredi wrote:
> On Thu, Sep 26, 2013 at 5:34 PM, J. Bruce Fields  
> <bfields@fieldses.org> wrote:
> > On Thu, Sep 26, 2013 at 10:58:05AM +0200, Miklos Szeredi wrote:
> >> On Wed, Sep 25, 2013 at 11:07 PM, Zach Brown <zab@redhat.com>  
> wrote:
> >> >> A client-side copy will be slower, but I guess it does have the
> >> >> advantage that the application can track progress to some  
> degree, and
> >> >> abort it fairly quickly without leaving the file in a totally  
> undefined
> >> >> state--and both might be useful if the copy's not a simple  
> constant-time
> >> >> operation.
> >> >
> >> > I suppose, but can't the app achieve a nice middle ground by  
> copying the
> >> > file in smaller syscalls?  Avoid bulk data motion back to the  
> client,
> >> > but still get notification every, I dunno, few hundred meg?
> >>
> >> Yes.  And if "cp"  could just be switched from a read+write syscall
> >> pair to a single splice syscall using the same buffer size.
> >
> > Will the various magic fs-specific copy operations become  
> inefficient
> > when the range copied is too small?
> 
> We could treat spice-copy operations just like write operations (can
> be buffered, coalesced, synced).
> 
> But I'm not sure it's worth the effort; 99% of the use of this
> interface will be copying whole files.

My "patch" implementation (in busybox and toybox) hits a point where it  
wants to copy the rest of the file, once there are no more hunks to  
apply. This is not copying a whole file. A similar thing happens with  
tail when you use the +N syntax to skip start instead of end lines. I  
can see sed doing a similar thing when told to operate on line ranges...

Note sure your 99% holds up here.

Rob