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aa6f4e51a7
Introduces new libdevmapper function dm_tree_node_add_snapshot_merge_target Verifies that the kernel (dm-snapshot) provides the 'snapshot-merge' target. Activate origin LV as snapshot-merge target. Using snapshot-origin target would be pointless because the origin contains volatile data while a merge is in progress. Because snapshot-merge target is activated in place of the snapshot-origin target it must be resumed after all other snapshots (just like snapshot-origin does) --- otherwise small window for data corruption would exist. Ideally the merging snapshot would not be activated at all but if it is to be activated (because snapshot was already active) it _must_ be done after the snapshot-merge. This insures that DM's snapshot-merge target will perform exception handover in the proper order (new->resume before old->resume). DM's snapshot-merge does support handover if the reverse sequence is used (old->resume before new->resume) but DM will fail to resume the old snapshot; leaving it suspended. To insure the proper activation sequence dm_tree_activate_children() was updated to accommodate an additional 'activation_priority' level. All regular snapshots are 0, snapshot-merge is 1, and merging snapshot is 2.