-----Original Message-----
From: Benny Halevy [mailto:bhalevy.lists@xxxxxxxxx] On Behalf Of Benny
Halevy
Sent: Thursday, July 08, 2010 12:00 PM
To: Trond Myklebust
Cc: Black, David; Noveck, David; Muntz, Daniel;
linux-nfs@xxxxxxxxxxxxxxx; garth@xxxxxxxxxxx;
welch@xxxxxxxxxxx; nfsv4@xxxxxxxx; andros@xxxxxxxxxx
Subject: Re: [nfsv4] 4.1 client - LAYOUTCOMMIT & close
On Jul. 08, 2010, 2:14 +0300, Trond Myklebust
<trond.myklebust@xxxxxxxxxx> wrote:
> On Wed, 2010-07-07 at 19:09 -0400, Trond Myklebust wrote:
>> On Wed, 2010-07-07 at 18:52 -0400, Trond Myklebust wrote:
>>> On Wed, 2010-07-07 at 18:44 -0400, david.black@xxxxxxx wrote:
>>>> Let me try this ...
>>>>
>>>> A correct client will always send LAYOUTCOMMIT.
>>>> Assume that the client is correct.
>>>> Hence if the LAYOUTCOMMIT doesn't arrive, something's failed.
>>>>
>>>> Important implication: No LAYOUTCOMMIT is an error/failure case.
It
>>>> just has to work; it doesn't have to be fast.
>>>>
Note that a LAYOUTRETURN can arrive without LAYOUTCOMMIT if the client
hasn't
written to the file. I'm not sure what about the blocks case though,
do you
implicitly free up any provisionally allocated blocks that the client
had not
explicitly committed using LAYOUTCOMMIT?
>>>> Suggestion: If a client dies while holding writeable layouts that
permit
>>>> write-in-place, and the client doesn't reappear or doesn't reclaim
those
>>>> layouts, then the server should assume that the files involved were
>>>> written before the client died, and set the file attributes
accordingly
>>>> as part of internally reclaiming the layout that the client has
>>>> abandoned.
Of course. That's part of the server recovery.
>>>>
>>>> Caveat: It may take a while for the server to determine that the
client
>>>> has abandoned a layout.
That's two lease times after a respective CB_LAYOUTRECALL.
>>>>
>>>> This can result in false positives (file appears to be modified
when it
>>>> wasn't) but won't yield false negatives (file does not appear to be
>>>> modified even though it was modified).
>>>
>>> OK... So we're going to have to turn off client side file caching
>>> entirely for pNFS? I can do that...
>>>
>>> The above won't work. Think readahead...
>>
>> So... What can work, is if you modify it to work explicitly for
>> close-to-open
>>
>> "Upon receiving an OPEN, LOCK or a WANT_DELEGATION, the server must
>> check that it has received LAYOUTCOMMITs from any other clients that
may
>> have the file open for writing. If it hasn't, then it MUST take some
>> action to ensure that any file data changes are accompanied by a
change
> ^ potentially visible
>> attribute update."
That should be OK as long as it's not for every GETATTR for the change,
mtime,
or size attributes.
>>
>> Then you can add the above suggestion without the offending caveat.
Note
>> however that it does break the "SHOULD NOT" admonition in section
>> 18.32.4.
Better be safe than sorry in this rare error case.
Benny
>>
>> Trond
>>
>>
>>> Trond
>>>
>>>> Thanks,
>>>> --David
>>>>
>>>>> -----Original Message-----
>>>>> From: nfsv4-bounces@xxxxxxxx [mailto:nfsv4-bounces@xxxxxxxx] On
Behalf
>>>> Of Noveck_David@xxxxxxx
>>>>> Sent: Wednesday, July 07, 2010 6:04 PM
>>>>> To: Trond.Myklebust@xxxxxxxxxx; Muntz, Daniel
>>>>> Cc: linux-nfs@xxxxxxxxxxxxxxx; garth@xxxxxxxxxxx;
welch@xxxxxxxxxxx;
>>>> nfsv4@xxxxxxxx;
>>>>> andros@xxxxxxxxxx; bhalevy@xxxxxxxxxxx
>>>>> Subject: Re: [nfsv4] 4.1 client - LAYOUTCOMMIT & close
>>>>>
>>>>>> Yes. I would agree that the client cannot rely on the updates
being
>>>> made
>>>>>> visible if it fails to send the LAYOUTCOMMIT. My point was simply
>>>> that a
>>>>>> compliant server MUST also have a valid strategy for dealing with
>>>> the
>>>>>> case where the client doesn't send it.
>>>>>
>>>>> So you are saying the updates "MUST be made visible" through the
>>>>> server's valid strategy. Is that right.
>>>>>
>>>>> And that the client cannot rely on that. Why not, if the server
must
>>>>> have a valid strategy.
>>>>>
>>>>> Is this just prudent "belt and suspenders" design or what?
>>>>>
>>>>> It seems to me that if one side here is MUST (and the spec needs
to be
>>>>> clearer about what might or might not constitute a valid
strategy),
>>>> then
>>>>> the other side should be SHOULD.
>>>>>
>>>>> If both sides are "MUST", then if things don't work out then the
>>>> client
>>>>> and server can equally point to one another and say "It's his
fault".
>>>>>
>>>>> Am I missing something here?
>>>>>
>>>>>
>>>>>
>>>>> -----Original Message-----
>>>>> From: nfsv4-bounces@xxxxxxxx [mailto:nfsv4-bounces@xxxxxxxx] On
Behalf
>>>>> Of Trond Myklebust
>>>>> Sent: Wednesday, July 07, 2010 5:01 PM
>>>>> To: Muntz, Daniel
>>>>> Cc: linux-nfs@xxxxxxxxxxxxxxx; garth@xxxxxxxxxxx;
welch@xxxxxxxxxxx;
>>>>> nfsv4@xxxxxxxx; andros@xxxxxxxxxx; bhalevy@xxxxxxxxxxx
>>>>> Subject: Re: [nfsv4] 4.1 client - LAYOUTCOMMIT & close
>>>>>
>>>>> On Wed, 2010-07-07 at 16:39 -0400, Daniel.Muntz@xxxxxxx wrote:
>>>>>> To bring this discussion full circle, since we agree that a
>>>> compliant
>>>>>> server can implement a scheme where written data does not become
>>>>> visible
>>>>>> until after a LAYOUTCOMMIT, do we also agree that LAYOUTCOMMIT
is a
>>>>>> "MUST" from a compliant client (independent of layout type)?
>>>>>
>>>>> Yes. I would agree that the client cannot rely on the updates
being
>>>> made
>>>>> visible if it fails to send the LAYOUTCOMMIT. My point was simply
that
>>>> a
>>>>> compliant server MUST also have a valid strategy for dealing with
the
>>>>> case where the client doesn't send it.
>>>>>
>>>>> Cheers
>>>>> Trond
>>>>>
>>>>>> -Dan
>>>>>>
>>>>>>> -----Original Message-----
>>>>>>> From: nfsv4-bounces@xxxxxxxx [mailto:nfsv4-bounces@xxxxxxxx]
>>>>>>> On Behalf Of Trond Myklebust
>>>>>>> Sent: Wednesday, July 07, 2010 7:04 AM
>>>>>>> To: Benny Halevy
>>>>>>> Cc: andros@xxxxxxxxxx; linux-nfs@xxxxxxxxxxxxxxx; Garth
>>>>>>> Gibson; Brent Welch; NFSv4
>>>>>>> Subject: Re: [nfsv4] 4.1 client - LAYOUTCOMMIT & close
>>>>>>>
>>>>>>> On Wed, 2010-07-07 at 16:51 +0300, Benny Halevy wrote:
>>>>>>>> On Jul. 07, 2010, 16:18 +0300, Trond Myklebust
>>>>>>> <Trond.Myklebust@xxxxxxxxxx> wrote:
>>>>>>>>> On Wed, 2010-07-07 at 09:06 -0400, Trond Myklebust wrote:
>>>>>>>>>> On Wed, 2010-07-07 at 15:05 +0300, Benny Halevy wrote:
>>>>>>>>>>> On Jul. 06, 2010, 23:40 +0300, Trond Myklebust
>>>>>>> <trond.myklebust@xxxxxxxxxx> wrote:
>>>>>>>>>>>> On Tue, 2010-07-06 at 15:20 -0400, Daniel.Muntz@xxxxxxx
>>>>> wrote:
>>>>>>>>>>>>> The COMMIT to the DS, ttbomk, commits data on the DS. I
>>>> see it as
>>>>>>>>>>>>> orthogonal to updating the metadata on the MDS (but
>>>> perhaps I'm wrong).
>>>>>>>>>>>>> As sjoshi@bluearc mentioned, the LAYOUTCOMMIT provides a
>>>> synchronization
>>>>>>>>>>>>> point, so even if the non-clustered server does not want
>>>> to update
>>>>>>>>>>>>> metadata on every DS I/O, the LAYOUTCOMMIT could also be a
>>>> trigger to
>>>>>>>>>>>>> execute whatever synchronization mechanism the implementer
>>>> wishes to put
>>>>>>>>>>>>> in the control protocol.
>>>>>>>>>>>>
>>>>>>>>>>>> As far as I'm aware, there are no exceptions in RFC5661
>>>> that would allow
>>>>>>>>>>>> pNFS servers to break the rule that any visible change to
>>>> the data must
>>>>>>>>>>>> be atomically accompanied with a change attribute update.
>>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> Trond, I'm not sure how this rule you mentioned is
>>>> specified.
>>>>>>>>>>>
>>>>>>>>>>> See more in section 12.5.4 and 12.5.4.1. LAYOUTCOMMIT and
>>>> change/time_modify
>>>>>>>>>>> in particular:
>>>>>>>>>>>
>>>>>>>>>>> For some layout protocols, the storage device is able to
>>>> notify the
>>>>>>>>>>> metadata server of the occurrence of an I/O; as a result,
>>>> the change
>>>>>>>>>>> and time_modify attributes may be updated at the metadata
>>>> server.
>>>>>>>>>>> For a metadata server that is capable of monitoring
>>>> updates to the
>>>>>>>>>>> change and time_modify attributes, LAYOUTCOMMIT
>>>> processing is not
>>>>>>>>>>> required to update the change attribute. In this case,
>>>> the metadata
>>>>>>>>>>> server must ensure that no further update to the data has
>>>> occurred
>>>>>>>>>>> since the last update of the attributes; file-based
>>>> protocols may
>>>>>>>>>>> have enough information to make this determination or may
>>>> update the
>>>>>>>>>>> change attribute upon each file modification. This also
>>>> applies for
>>>>>>>>>>> the time_modify attribute. If the server implementation
>>>> is able to
>>>>>>>>>>> determine that the file has not been modified since the
>>>> last
>>>>>>>>>>> time_modify update, the server need not update
>>>> time_modify at
>>>>>>>>>>> LAYOUTCOMMIT. At LAYOUTCOMMIT completion, the updated
>>>> attributes
>>>>>>>>>>> should be visible if that file was modified since the
>>>> latest previous
>>>>>>>>>>> LAYOUTCOMMIT or LAYOUTGET
>>>>>>>>>>
>>>>>>>>>> I know. However the above paragraph does not state that the
>>>> server
>>>>>>>>>> should make those changes visible to clients other than the
>>>> one that is
>>>>>>>>>> writing.
>>>>>>>>>>
>>>>>>>>>> Section 18.32.4 states that writes will cause the
>>>> time_modified and
>>>>>>>>>> change attributes to be updated (if and only if the file data
>>>> is
>>>>>>>>>> modified). Several other sections rely on this behaviour,
>>>> including
>>>>>>>>>> section 10.3.1, section 11.7.2.2, and section 11.7.7.
>>>>>>>>>>
>>>>>>>>>> The only 'special behaviour' that I see allowed for pNFS is
>>>> in section
>>>>>>>>>> 13.10, which states that clients can't expect to see changes
>>>>>>>>>> immediately, but that they must be able to expect
>>>> close-to-open
>>>>>>>>>> semantics to work. Again, if this is to be the case, then the
>>>> server
>>>>>>>>>> _must_ be able to deal with the case where client 1 dies
>>>> before it can
>>>>>>>>>> issue the LAYOUTCOMMIT.
>>>>>>>>
>>>>>>>> Agreed.
>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>>> As I see it, if your server allows one client to read data
>>>> that may have
>>>>>>>>>>>> been modified by another client that holds a WRITE layout
>>>> for that range
>>>>>>>>>>>> then (since that is a visible data change) it should
>>>> provide a change
>>>>>>>>>>>> attribute update irrespective of whether or not a
>>>> LAYOUTCOMMIT has been
>>>>>>>>>>>> sent.
>>>>>>>>>>>
>>>>>>>>>>> the requirement for the server in WRITE's implementation
>>>> section
>>>>>>>>>>> is quite weak: "It is assumed that the act of writing data
>>>> to a file will
>>>>>>>>>>> cause the time_modified and change attributes of the file to
>>>> be updated."
>>>>>>>>>>>
>>>>>>>>>>> The difference here is that for pNFS the written data is not
>>>> guaranteed
>>>>>>>>>>> to be visible until LAYOUTCOMMIT. In a broader sense,
>>>> assuming the clients
>>>>>>>>>>> are caching dirty data and use a write-behind cache,
>>>> application-written data
>>>>>>>>>>> may be visible to other processes on the same host but not
>>>> to others until
>>>>>>>>>>> fsync() or close() - open-to-close semantics are the only
>>>> thing the client
>>>>>>>>>>> guarantees, right? Issuing LAYOUTCOMMIT on fsync() and
>>>> close() ensure the
>>>>>>>>>>> data is committed to stable storage and is visible to all
>>>> other clients in
>>>>>>>>>>> the cluster.
>>>>>>>>>>
>>>>>>>>>> See above. I'm not disputing your statement that 'the written
>>>> data is
>>>>>>>>>> not guaranteed to be visible until LAYOUTCOMMIT'. I am
>>>> disputing an
>>>>>>>>>> assumption that 'the written data may be visible without an
>>>> accompanying
>>>>>>>>>> change attribute update'.
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> In other words, I'd expect the following scenario to give the
>>>> same
>>>>>>>>> results in NFSv4.1 w/pNFS as it does in NFSv4:
>>>>>>>>
>>>>>>>> That's a strong requirement that may limit the scalability of
>>>> the server.
>>>>>>>>
>>>>>>>> The spirit of the pNFS operations, at least from Panasas
>>>> perspective was that
>>>>>>>> the data is transient until LAYOUTCOMMIT, meaning it may or may
>>>> not be visible
>>>>>>>> to clients other than the one who wrote it, and its associated
>>>> metadata MUST
>>>>>>>> be updated and describe the new data only on LAYOUTCOMMIT and
>>>> until then it's
>>>>>>>> undefined, i.e. it's up to the server implementation whether to
>>>> update it or not.
>>>>>>>>
>>>>>>>> Without locking, what do the stronger semantics buy you?
>>>>>>>> Even if a client verified the change_attribute new data may
>>>> become visible
>>>>>>>> at any time after the GETATTR if the file/byte range aren't
>>>> locked.
>>>>>>>
>>>>>>> There is no locking needed in the scenario below: it is ordinary
>>>>>>> close-to-open semantics.
>>>>>>>
>>>>>>> The point is that if you remove the one and only way that
clients
>>>> have
>>>>>>> to determine whether or not their data caches are valid, then
they
>>>> can
>>>>>>> no longer cache data at all, and server scalability will be shot
>>>> to
>>>>>>> smithereens anyway.
>>>>>>>
>>>>>>> Trond
>>>>>>>
>>>>>>>> Benny
>>>>>>>>
>>>>>>>>>
>>>>>>>>> Client 1 Client 2
>>>>>>>>> ======== ========
>>>>>>>>>
>>>>>>>>> OPEN foo
>>>>>>>>> READ
>>>>>>>>> CLOSE
>>>>>>>>> OPEN
>>>>>>>>> LAYOUTGET ...
>>>>>>>>> WRITE via DS
>>>>>>>>> <dies>...
>>>>>>>>> OPEN foo
>>>>>>>>> verify change_attr
>>>>>>>>> READ if above WRITE is visible
>>>>>>>>> CLOSE
>>>>>>>>>
>>>>>>>>> Trond
>>>>>>>>> _______________________________________________
>>>>>>>>> nfsv4 mailing list
>>>>>>>>> nfsv4@xxxxxxxx
>>>>>>>>> https://www.ietf.org/mailman/listinfo/nfsv4
>>>>>>>
>>>>>>>
>>>>>>> _______________________________________________
>>>>>>> nfsv4 mailing list
>>>>>>> nfsv4@xxxxxxxx
>>>>>>> https://www.ietf.org/mailman/listinfo/nfsv4
>>>>>>>
>>>>>>>
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> nfsv4 mailing list
>>>>> nfsv4@xxxxxxxx
>>>>> https://www.ietf.org/mailman/listinfo/nfsv4
>>>>>
>>>>> _______________________________________________
>>>>> nfsv4 mailing list
>>>>> nfsv4@xxxxxxxx
>>>>> https://www.ietf.org/mailman/listinfo/nfsv4
>>>>
>>>
>>>
>>
>>
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