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. 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 -- 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
