Re: [RFC 4/5] NFSD: Defer copying

From: Bryan Schumaker <bjschuma@netapp.com>
To: "J. Bruce Fields" <bfields@fieldses.org>
Cc: <Trond.Myklebust@netapp.com>, <linux-nfs@vger.kernel.org>
Subject: Re: [RFC 4/5] NFSD: Defer copying
Date: Mon, 22 Jul 2013 15:37:00 -0400	[thread overview]
Message-ID: <51ED89DC.7050406@netapp.com> (raw)
In-Reply-To: <20130722193000.GD10109@fieldses.org>

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.

- Bryan

> 
>>> 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.
> 