default void preClearRegionBlockCache(ObserverContext<RegionServerCoprocessorEnvironment> ctx)
    throws IOException {
  }
Called before clearing the block caches for one or more regions
@param ctx the coprocessor instance's environment
@throws IOException if you need to signal an IO error
hbase-server/src/main/java/org/apache/hadoop/hbase/coprocessor/RegionServerObserver.java