code: private boolean set_cache( long cache ) {
      while( true ) { // Spin till get it
        long old = _cache; // Read once at the start
        if( !H2O.larger(cloud(cache),cloud(old)) ) // Rolling backwards?
          // Attempt to set for an older Cloud. Blow out with a failure; caller
          // should retry on a new Cloud.
          return false;
        assert cloud(cache) != cloud(old) || cache == old;
        if( old == cache ) return true; // Fast-path cutout
        if( _cacheUpdater.compareAndSet(this,old,cache) ) return true;
        // Can fail if the cache is really old, and just got updated to a version
        // which is still not the latest, and we are trying to update it again.
      }
    }
docs: Update the cache, but only to strictly newer Clouds