 // We allow rollback to a state, which is either consistent with
    // the namespace state or can be further upgraded to it.
    // In another word, we can only roll back when ( storedLV >= software LV)
    // && ( DN.previousCTime <= NN.ctime)
 // We allow rollback to a state, which is either consistent with
    // the namespace state or can be further upgraded to it.
    // In another word, we can only roll back when ( storedLV >= software LV)
    // && ( DN.previousCTime <= NN.ctime) | BlockPoolSliceStorage.java