private synchronized void removeBucket(Bucket b) {
      assert b.isCompletelyFree();
      bucketList.remove(b);
      freeBuckets.remove(b);
      completelyFreeBuckets.remove(b);
    }
Find a bucket to allocate a block
@return the offset in the IOEngine
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/bucket/BucketAllocator.java