Date: Mon, 22 Jul 2013 15:55:56 -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: <20130722195556.GG10109@fieldses.org>
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On Mon, Jul 22, 2013 at 03:54:00PM -0400, Bryan Schumaker wrote:
> On 07/22/2013 03:43 PM, J. Bruce Fields wrote:
> > On Mon, Jul 22, 2013 at 03:37:00PM -0400, Bryan Schumaker wrote:
> >> On 07/22/2013 03:30 PM, J. Bruce Fields wrote:
> >>> 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.
> >>
> >> Fair enough.  I'll think it out and do something!  Easy solution: save this patch for later and only support the sync version of copy for the final version of this patch series.
> > 
> > I can't remember--does the spec give the server a clear way to bail out
> > and tell the client to fall back on a normal copy in cases where the
> > server knows the copy could take an unreasonable amount of time?
> > 
> > --b.
> 
> I don't think so.  Is there ever a case where copying over the network would be slower than copying on the server?

Mybe not, but if the copy will take a minute, then we don't want to tie
up an rpc slot for a minute.

--b.