On Thu, May 28, 2020 at 8:44 AM Hans van den Bogert <hansbogert@xxxxxxxxx> wrote: > Hi list, > > When reading the documentation for the new way of mirroring [1], some > questions arose, especially with the following sentence: > > > Since this mode is not point-in-time consistent, the full snapshot > delta will need to be synced prior to use during a failover scenario. > > 1) I'm not sure I follow. Why are snapshots not point-in-time, whereas > using the journal-based mirroring is? > I agree that it's probably a poor choice of verbiage. Both are technically crash / point-in-time consistent. The difference is that journal-based mirroring provides a replayable stream of updates that are synced with a very fine granularity (down to single IO updates) whereas snapshot-based mirroring has a very coarse granularity. 2) Also, I am not sure what the implications are for the second part of > the referenced sentence, what significance does the word "full" bear in > "full snapshot delta" ? Given mirroring snapshot A and B, the full delta of changes between snapshots A and B will need to be successfully synced / applied to the non-primary image. If you force a failover before a the delta between snapshot A and B can be applied, the system will rollback the non-primary image to snapshot A to get the non-primary image to a consistent state. > Further, Isn't it obvious that in order to do a > failover, the other side should have the latest snapshot? - I am > probably missing what the documentation is trying to tell me. > Not all failovers are planned. All in all, I think some background concepts are missing in order to > read this chapter of the documentation. It might well be that this is > explained somewhere else in the docs, but maybe we should link to that. > > If someone can explain the above 2 points, I'm willing to take a shot at > making a patch if I can make this clearer in the docs. > Please feel free to submit any clarifying updates. > > Thanks, > > Hans > > > [1] https://docs.ceph.com/docs/octopus/rbd/rbdcourse-mirroring/ > <https://docs.ceph.com/docs/octopus/rbd/rbd-mirroring/> > _______________________________________________ > ceph-users mailing list -- ceph-users@xxxxxxx > To unsubscribe send an email to ceph-users-leave@xxxxxxx > > -- Jason _______________________________________________ ceph-users mailing list -- ceph-users@xxxxxxx To unsubscribe send an email to ceph-users-leave@xxxxxxx
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