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. > > 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. > > Caveat: It may take a while for the server to determine that the client > has abandoned a layout. > > 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... 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 > -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
