为何profile里面的IO值为0 ？？？？？？？？？？

操作系统:CENTOS 5.5
MYSQL:5.1.56 RPM
PROFILE如下：
SQL code

*************************** 14. row ***************************
             Status: Copying to tmp table on disk
           Duration: 210.638293
           CPU_user: 211.342871
         CPU_system: 1.418785
  Context_voluntary: 1718
Context_involuntary: 534
       Block_ops_in: 0
      Block_ops_out: 0
      Messages_sent: 0
  Messages_received: 0
  Page_faults_major: 0
  Page_faults_minor: 21160
              Swaps: 0
    Source_function: unknown function
        Source_file: sql_select.cc
        Source_line: 11041

状态时Copying to tmp table on disk，进行了磁盘临时表，
Block_ops_in: 0
Block_ops_out: 0
这2个值应该是有值的吧。而且我观察了很久，这个值不管什么查询，都是0.

SQL code

  Copying to tmp table           |  29.165256 |  29.363536 |   0.187971 |       
  converting HEAP to MyISAM      |   0.163705 |   0.053992 |   0.109983 |        
  Copying to tmp table on disk   | 210.638293 | 211.342871 |   1.418785 |         

其他步骤如上。