【轉載】latch: cache buffers chains，latchbuffers

【轉載】latch: cache buffers chains，latchbuffers

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    當一個數據塊讀入sga區，相應的buffer header會被放置到hash列表上,我們稱其這hash chains,chain在中文的意為鏈條或串的意思，表達就是關連性.如果一個進程想訪問或修改hash chain上的block,它首先要獲得”cache buffers chains” latch。

原因一：低效率的SQL語句（主要體現在邏輯讀過高）

cache buffers chains latch很大程度與邏輯讀有關，所以要觀注v$sql中BUFFER_GETS/EXECUTIONS大的語句。
同時每一個邏輯讀需要一個latch get 操作及一個cpu操作，這樣的sql也會很耗cpu資源。

原因二：熱塊（訪問過於頻繁）

找出熱點塊方法一：

--找出p1raw

select p1,p1raw from v$session_wait where event='latch: cache buffers chains';

--找到對象

SELECT /*+ RULE */

 E.OWNER || '.' || E.SEGMENT_NAME SEGMENT_NAME,

 E.PARTITION_NAME,

 E.EXTENT_ID EXTENT#,

 X.DBABLK - E.BLOCK_ID + 1 BLOCK#,

 X.TCH,

 L.CHILD#

  FROM SYS.V$LATCH_CHILDREN L, SYS.X$BH X, SYS.DBA_EXTENTS E

 WHERE X.HLADDR = '00000002576EE018'--p1raw

   AND E.FILE_ID = X.FILE#

   AND X.HLADDR = L.ADDR

   AND X.DBABLK BETWEEN E.BLOCK_ID AND E.BLOCK_ID + E.BLOCKS - 1

 ORDER BY X.TCH DESC;

找出熱點塊方法二：

--直接找出熱點塊

SELECT OBJECT_NAME, SUBOBJECT_NAME

  FROM DBA_OBJECTS

 WHERE DATA_OBJECT_ID IN

       (SELECT DATA_OBJECT_ID

          FROM (SELECT OBJ DATA_OBJECT_ID, FILE#, DBABLK, CLASS, STATE, TCH

                  FROM X$BH

                 WHERE HLADDR IN (SELECT ADDR

                                    FROM (SELECT ADDR

                                            FROM V$LATCH_CHILDREN

                                           ORDER BY (GETS + MISSES + SLEEPS) DESC)

                                   WHERE ROWNUM < 10)

                 ORDER BY TCH DESC)

         WHERE ROWNUM < 10);

問oracle數據庫等待事件latch:cache buffer chains是為何？看aw

pmon長期持有cache buffers chains導致實例hang住一例
www.askmaclean.com/...B.html