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Don Burleson Blog 







Oracle RAC blocking sessions

Oracle Tips by Burleson Consulting
May 26,  2009

Question:  How to I find Oracle blocking sessions in RAC?

Answer:  You can query the gv$lock and gv$session views to locate a blocking session in RAC.  Killing a session in RAC is different than killing an ordinary blocking session, and dba_blockers and dba_waiters cannot always help identify blocking RAC sessions. 

For full scripts, download the Oracle script collection.

Miladin Modrakovic offers this script to detect and kill RAC blocking sessions, using gv$session and gv$lock:

   sqlstmt   VARCHAR2 (1000);
   FOR x IN (SELECT gvh.SID sessid, gvs.serial# serial,
                    gvh.inst_id instance_id
               FROM gv$lock gvh, gv$lock gvw, gv$session gvs
              WHERE (gvh.id1, gvh.id2) IN (SELECT id1, id2
                                             FROM gv$lock
                                            WHERE request = 0
                                           SELECT id1, id2
                                             FROM gv$lock
                                            WHERE lmode = 0)
                AND gvh.id1 = gvw.id1
                AND gvh.id2 = gvw.id2
                AND gvh.request = 0
                AND gvw.lmode = 0
                AND gvh.SID = gvs.SID
                AND gvh.inst_id = gvs.inst_id)
      sqlstmt :=
         || x.sessid
         || ‘,’
         || x.serial
         || ‘,@’
         || x.instance_id
         || ””;
      DBMS_OUTPUT.put_line (sqlstmt);
      EXECUTE IMMEDIATE sqlstmt;
   END kill_blovk;
When you run this script it will generate the alter system kill session syntax for the RAC blocking session: 
SQL> set serveroutput on
SQL> exec kill_blocker;
PL/SQL procedure successfully completed.   

Also see these related notes on Oracle blocking sessions:

SELECT DECODE (l.BLOCK, 0, 'Waiting', 'Blocking ->') user_status
,CHR (39) || s.SID || ',' || s.serial# || CHR (39) sid_serial
,(SELECT instance_name FROM gv$instance WHERE inst_id = l.inst_id)
,DECODE (l.TYPE,'RT', 'Redo Log Buffer','TD', 'Dictionary'
,'TM', 'DML','TS', 'Temp Segments','TX', 'Transaction'
,'UL', 'User','RW', 'Row Wait',l.TYPE) lock_type
,DECODE (l.lmode,0, 'None',1, 'Null',2, 'Row Share',3, 'Row Excl.'
,4, 'Share',5, 'S/Row Excl.',6, 'Exclusive'
,LTRIM (TO_CHAR (lmode, '990'))) lock_mode
--,DECODE(l.BLOCK, 0, 'Not Blocking', 1, 'Blocking', 2, 'Global') lock_status
   gv$lock l

   gv$session s

ON (l.inst_id = s.inst_id
AND l.SID = s.SID)
JOIN gv$locked_object o
ON (o.inst_id = s.inst_id
AND s.SID = o.session_id)
JOIN dba_objects d
ON (d.object_id = o.object_id)
WHERE (l.id1, l.id2, l.TYPE) IN (SELECT id1, id2, TYPE
FROM gv$lock
WHERE request > 0)
ORDER BY id1, id2, ctime DESC;

Also see these notes on RAC blocking sessions detection:

find blocking sessions with v$session

find the data block for a blocking session

Oracle Grid and Real Application Clusters

See working examples of Oracle Grid and RAC in the book Oracle Grid and Real Application Clusters.

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