rbd: detect when clone image is flattened
A format 2 clone image can be the subject of a "flatten" operation, during which all of its data gets "copied up" from its parent image, leaving the image fully populated. Once this is complete, the clone's association with the parent is abolished. Since this can occur when a clone is mapped, we need to detect when it has occurred and handle it accordingly. We know an image has been flattened when we know it at one time had a parent, but we have learned (via a "get_parent" object class method call) it no longer has one. There might be in-flight requests at the point we learn an image has been flattened, so we can't simply clean up parent data structures right away. Instead, we'll drop the initial parent reference when the parent has disappeared (rather than when the image gets destroyed), which will allow the last in-flight reference to clean things up when it's complete. We leverage the fact that a zero parent overlap renders an image effectively unlayered. We set the overlap to 0 at the point we detect the clone image has flattened, which allows the unlayered behavior to take effect immediately, while keeping other parent structures in place until in-flight requests to complete. This and the next few patches resolve: http://tracker.ceph.com/issues/3763Signed-off-by: Alex Elder <elder@inktank.com> Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
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