Date: Mon, 22 Jul 2013 15:30:00 -0400
From: "J. Bruce Fields" <bfields@fieldses.org>
To: Bryan Schumaker <bjschuma@netapp.com>
Cc: Trond.Myklebust@netapp.com, linux-nfs@vger.kernel.org
Subject: Re: [RFC 4/5] NFSD: Defer copying
Message-ID: <20130722193000.GD10109@fieldses.org>
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 <51ED8549.3040308@netapp.com>
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On Mon, Jul 22, 2013 at 03:17:29PM -0400, Bryan Schumaker wrote:
> On 07/22/2013 02:50 PM, J. Bruce Fields wrote:
> > On Fri, Jul 19, 2013 at 05:03:49PM -0400, bjschuma@netapp.com wrote:
> >> From: Bryan Schumaker <bjschuma@netapp.com>
> >>
> >> Rather than performing the copy right away, schedule it to run later and
> >> reply to the client.  Later, send a callback to notify the client that
> >> the copy has finished.
> > 
> > I believe you need to implement the referring triple support described
> > in http://tools.ietf.org/html/rfc5661#section-2.10.6.3 to fix the race
> > described in
> > http://tools.ietf.org/html/draft-ietf-nfsv4-minorversion2-19#section-15.1.3
> > .
> 
> I'll re-read and re-write.
> 
> > 
> > I see cb_delay initialized below, but not otherwise used.  Am I missing
> > anything?
> 
> Whoops!  I was using that earlier to try to fake up a callback, but I eventually decided it's easier to just do the copy asynchronously.  I must have forgotten to take it out :(
> 
> > 
> > What about OFFLOAD_STATUS and OFFLOAD_ABORT?
> 
> I haven't thought out those too much... I haven't thought about a use for them on the client yet.

If it might be a long-running copy, I assume the client needs the
ability to abort if the caller is killed.

(Dumb question: what happens on the network partition?  Does the server
abort the copy when it expires the client state?)

In any case,
http://tools.ietf.org/html/draft-ietf-nfsv4-minorversion2-19#section-15.1.3
says "If a server's COPY operation returns a stateid, then the server
MUST also support these operations: CB_OFFLOAD, OFFLOAD_ABORT, and
OFFLOAD_STATUS."

So even if we've no use for them on the client then we still need to
implement them (and probably just write a basic pynfs test).  Either
that or update the spec.

> > In some common cases the reply will be very quick, and we might be
> > better off handling it synchronously.  Could we implement a heuristic
> > like "copy synchronously if the filesystem has special support or the
> > range is less than the maximum iosize, otherwise copy asynchronously"?
> 
> I'm sure that can be done, I'm just not sure how to do it yet...

OK, thanks.

--b.