• Lars Ellenberg's avatar
    drbd: introduce unfence-peer handler · 26a96110
    Lars Ellenberg authored
    When resync is finished, we already call the "after-resync-target"
    handler (on the former sync target, obviously), once per volume.
    
    Paired with the before-resync-target handler, you can create snapshots,
    before the resync causes the volumes to become inconsistent,
    and discard those snapshots again, once they are no longer needed.
    
    It was also overloaded to be paired with the "fence-peer" handler,
    to "unfence" once the volumes are up-to-date and known good.
    
    This has some disadvantages, though: we call "fence-peer" for the whole
    connection (once for the group of volumes), but would call unfence as
    side-effect of after-resync-target once for each volume.
    
    Also, we fence on a (current, or about to become) Primary,
    which will later become the sync-source.
    
    Calling unfence only as a side effect of the after-resync-target
    handler opens a race window, between a new fence on the Primary
    (SyncTarget) and the unfence on the SyncTarget, which is difficult to
    close without some kind of "cluster wide lock" in those handlers.
    
    We would not need those handlers if we could still communicate.
    Which makes trying to aquire a cluster wide lock from those handlers
    seem like a very bad idea.
    
    This introduces the "unfence-peer" handler, which will be called
    per connection (once for the group of volumes), just like the fence
    handler, only once all volumes are back in sync, and on the SyncSource.
    
    Which is expected to be the node that previously called "fence", the
    node that is currently allowed to be Primary, and thus the only node
    that could trigger a new "fence" that could race with this unfence.
    
    Which makes us not need any cluster wide synchronization here,
    serializing two scripts running on the same node is trivial.
    Signed-off-by: default avatarPhilipp Reisner <philipp.reisner@linbit.com>
    Signed-off-by: default avatarLars Ellenberg <lars.ellenberg@linbit.com>
    Signed-off-by: default avatarJens Axboe <axboe@fb.com>
    26a96110
drbd_int.h 80.9 KB