Re: [nfsv4] 4.1 client - LAYOUTCOMMIT & close

[Trond Myklebust <[e-mail]>]

On Thu, 2010-07-08 at 16:30 -0400, [e-mail] wrote:
> > 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?
> 
> In principle, yes as the blocks are no longer promised to the client, although
> lazy evaluation of this is an obvious optimization.
> 
> > >> "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.
> 
> I concur with Benny on both of the above - in essence, the unrecovered client failure is a reason to potentially ignore the "SHOULD" (server can't know whether it actually ignored the "SHOULD", hence better safe than sorry).  We probably ought to find a someplace appropriate to add a paragraph or two explaining this in one of the 4.2 documents.

Right. I'm only interested in fixing the close-to-open case. The case of
general GETATTR calls might be nice to fix too, but it should not be
essential in order to ensure that well-behaved applications continue to
work as expected.

Note, however, that legacy support for stateless protocols like NFSv2
and NFSv3 may be problematic: there is no equivalent of OPEN, and so the
server may have to do the above check on all NFSPROC2_GETATTR,
NFSPROC3_GETATTR, NFSPROC2_LOOKUP and NFSPROC3_LOOKUP requests.

   Trond

> Thanks,
> --David
> 
> 
> > -----Original Message-----
> > From: Benny Halevy [mailto:[e-mail]] On Behalf Of Benny Halevy
> > Sent: Thursday, July 08, 2010 12:00 PM
> > To: Trond Myklebust
> > Cc: Black, David; Noveck, David; Muntz, Daniel; [e-mail]; [e-mail];
> > [e-mail]; [e-mail]; [e-mail]
> > Subject: Re: [nfsv4] 4.1 client - LAYOUTCOMMIT & close
> > 
> > On Jul. 08, 2010, 2:14 +0300, Trond Myklebust <[e-mail]> 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, [e-mail] 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: [e-mail] [mailto:[e-mail]] On Behalf
> > >>>> Of [e-mail]
> > >>>>> Sent: Wednesday, July 07, 2010 6:04 PM
> > >>>>> To: [e-mail]; Muntz, Daniel
> > >>>>> Cc: [e-mail]; [e-mail]; [e-mail];
> > >>>> [e-mail];
> > >>>>> [e-mail]; [e-mail]
> > >>>>> 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: [e-mail] [mailto:[e-mail]] On Behalf
> > >>>>> Of Trond Myklebust
> > >>>>> Sent: Wednesday, July 07, 2010 5:01 PM
> > >>>>> To: Muntz, Daniel
> > >>>>> Cc: [e-mail]; [e-mail]; [e-mail];
> > >>>>> [e-mail]; [e-mail]; [e-mail]
> > >>>>> Subject: Re: [nfsv4] 4.1 client - LAYOUTCOMMIT & close
> > >>>>>
> > >>>>> On Wed, 2010-07-07 at 16:39 -0400, [e-mail] 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: [e-mail] [mailto:[e-mail]]
> > >>>>>>> On Behalf Of Trond Myklebust
> > >>>>>>> Sent: Wednesday, July 07, 2010 7:04 AM
> > >>>>>>> To: Benny Halevy
> > >>>>>>> Cc: [e-mail]; [e-mail]; 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
> > >>>>>>> <[e-mail]> 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
> > >>>>>>> <[e-mail]> wrote:
> > >>>>>>>>>>>> On Tue, 2010-07-06 at 15:20 -0400, [e-mail]
> > >>>>> 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
> > >>>>>>>>> [e-mail]
> > >>>>>>>>> https://www.ietf.org/mailman/listinfo/nfsv4
> > >>>>>>>
> > >>>>>>>
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> > >>>>>
> > >>>>>
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