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标题：rac kill 大事物后回滚慢,smon等待DFS lock handle和enq: TX – contention

作者：惜分飞©版权所有[未经本人同意,不得以任何形式转载,否则有进一步追究法律责任的权利.]

Customer killed a large DELETE statement of a large table in instance 1 in high workload period in RAC environment of 2 instances.
Transaction recovery did not yet complete for more than 1 hour by querying V$FAST_START_TRANSACTIONS.
Update on the same table on instance 2 hung more than 1 hour.
Transaction recovery complete quickly after shutdown instance 2.
ASH shows high waiting on “enq: TX – row lock contention” and “transaction” without blocking session until instance 2 shutdown after DELETE session killed in instance 1

SQL> select instance_number,program,event,BLOCKING_INST_ID b_inst,count(session_id) session_cnt,
count(BLOCKING_SESSION) b_session_cnt,min(sample_time) start_time,max(sample_time) end_time,count(*) cnt
  2  from DBA_HIST_ACTIVE_SESS_HISTORY where sql_id='d1xg4v7rqx3kt' and instance_number=2
group by instance_number,program,event,BLOCKING_INST_ID
order by instance_number,start_time
  3    4    5  ;

INSTANCE_NUMBER PROGRAM            EVENT                         B_INST SESSION_CNT B_SESSION_CNT START_TIME END_TIME       CNT
--------------- ------------------ ----------------------------- ------ ----------- ------------- ---------- ---------- -------
              2 JDBC Thin Client   enq: TX - row lock contention             728939             0 19:20:34   21:04:25    728939
              2 JDBC Thin Client   transaction                               489837             0 20:12:44   21:04:25    489837

ASH shows that all sessions of the “enq: TX – row lock contention” without blocking session on instance 2 were waiting for the transaction of the Killed DELETE on instance 1

SQL> select instance_number,event,trunc(p2/65536) XIDUSN, trunc(mod(p2,65536)) XIDSLOT, 
p3 XIDSQN,count(session_id) session_cnt,count(BLOCKING_SESSION) b_session_cnt,min(sample_time) start_time,max(sample_time) end_time,count(*) cnt
from DBA_HIST_ACTIVE_SESS_HISTORY where sql_id='d1xg4v7rqx3kt' and instance_number=2 and 
to_char(sample_time,'HH24:MI') >= '20:12' AND BLOCKING_SESSION IS NULL AND EVENT='enq: TX - row lock contention'
group by instance_number,event,trunc(p2/65536) , trunc(mod(p2,65536)) , p3
order by start_time
;
  2    3    4    5

INSTANCE_NUMBER EVENT                            XIDUSN  XIDSLOT   XIDSQN SESSION_CNT B_SESSION_CNT START_TIME END_TIME      CNT
--------------- ------------------------------ -------- -------- -------- ----------- ------------- ---------- ---------- ------
              2 enq: TX - row lock contention       932       15   623055      717850             0 20:12:55   21:04:25   717850
SQL>

ASH shows that SMON of instance 1 was waiting for “DFS lock handle” and “enq: TX – contention” among DELETE session killed in instance 1 and instance 2 shutdown.

SQL> select instance_number,session_id,session_serial#,program,event,BLOCKING_SESSION b_sid,
BLOCKING_SESSION_SERIAL# b_serial,BLOCKING_INST_ID b_inst,min(sample_time) start_time,
max(sample_time) end_time,count(*) cnt
 from DBA_HIST_ACTIVE_SESS_HISTORY where upper(program) like '%SMON%'
group by instance_number,session_id,session_serial#,program,event,BLOCKING_SESSION ,BLOCKING_SESSION_SERIAL# ,BLOCKING_INST_ID
order by instance_number,start_time
;

INSTANCE_NUMBER SESSION_ID SESSION_SERIAL# PROGRAM                       EVENT                 B_SID  B_SERIAL B_INST START_TIME END_TIME   CNT
--------------- ---------- --------------- ----------------------------- --------------------- ----- --------- ------ ---------- ---------- ---
              1       3761               1 oracle@<instance_name> (SMON) DFS lock handle                              19:05:24   21:05:07   128
              1       3761               1 oracle@<instance_name> (SMON) enq: TX - contention                         20:14:31   21:04:47    50

The P1 of “DFS lock handle” shows that SMON was requesting TA enqueue through cross instance calls which is used for serializing operations on undo segments and undo tablespaces

SQL> col event for a20
select instance_number,sample_time,session_id,session_serial#,program,event,p1,p2,p3
  from DBA_HIST_ACTIVE_SESS_HISTORY where upper(program) like '%SMON%' AND EVENT='DFS lock handle' ;
  SQL>   2
INSTANCE_NUMBER SAMPLE_TIME     SESSION_ID SESSION_SERIAL# PROGRAM                       EVENT                      P1     P2     P3
--------------- --------------- ---------- --------------- ----------------------------- ---------------- ------------ ------ ------
              1 19:05:24              3761               1 oracle@<instance_name> (SMON) DFS lock handle    1413545989      3   4722
              1 20:12:27              3761               1 oracle@<instance_name> (SMON) DFS lock handle    1413545989      3   6668
              1 20:13:40              3761               1 oracle@<instance_name> (SMON) DFS lock handle    1413545989      3   5787
              1 20:13:50              3761               1 oracle@<instance_name> (SMON) DFS lock handle    1413545989      3   5580
              1 20:14:00              3761               1 oracle@<instance_name> (SMON) DFS lock handle    1413545989      3   5826
... ...
              1 21:04:57              3761               1 oracle@<instance_name> (SMON) DFS lock handle    1413545989      3   1347
              1 21:05:07              3761               1 oracle@<instance_name> (SMON) DFS lock handle    1413545989      3   4582

144 rows selected.

* All of P1 is 1413545989, converted to hexadecimal: ‭54410005‬, 54 ASCII code: T, 41 ASCII code: A => TA enqueue

The P2,P3 of “enq: TX – row lock contention” shows that SMON process was requesting TX enqueue of the UNDO block of the transaction of the Killed DELETE on instance 1.

SQL> select instance_number,session_id,session_serial#,program,event,trunc(p2/65536) XIDUSN, trunc(mod(p2,65536)) XIDSLOT, p3 XIDSQN,COUNT(1) CNT,min(sample_time) start_time,max(sample_time) end_time
from DBA_HIST_ACTIVE_SESS_HISTORY WHERE session_id =3761 and SESSION_SERIAL# = 1 and instance_number=1 AND event like 'enq: TX - contention%'
group by instance_number,session_id,session_serial#,program,event,trunc(p2/65536) , trunc(mod(p2,65536)) , p3
;
  2    3    4
INSTANCE_NUMBER SESSION_ID SESSION_SERIAL# PROGRAM                        EVENT                  XIDUSN  XIDSLOT   XIDSQN CNT START_TIME END_TIME
--------------- ---------- --------------- ------------------------------ -------------------- -------- -------- -------- --- ---------- ----------
              1       3761               1 oracle@<instance_name> (SMON)  enq: TX - contention      932       15   623055  50 20:14:31   21:04:47

SQL>
SQL> select instance_number,session_id,session_serial#,program,event,CURRENT_OBJ#, CURRENT_FILE#, CURRENT_BLOCK#,CURRENT_ROW#,COUNT(1) CNT,min(sample_time) start_time,max(sample_time) end_time
  2  from DBA_HIST_ACTIVE_SESS_HISTORY WHERE session_id =3761 and SESSION_SERIAL# = 1 and instance_number=1 AND event like 'enq: TX - contention%'
group by instance_number,session_id,session_serial#,program,event,CURRENT_OBJ#, CURRENT_FILE#, CURRENT_BLOCK#,CURRENT_ROW#
order by start_time
;
  3    4    5
INSTANCE_NUMBER SESSION_ID SESSION_SERIAL# PROGRAM                       EVENT                CURRENT_OBJ# CURRENT_FILE# CURRENT_BLOCK# CURRENT_ROW#   CNT START_TIME END_TIME
--------------- ---------- --------------- ----------------------------- -------------------- ------------ ------------- -------------- ------------ ----- ---------- ----------
              1       3761               1 oracle@<instance_name> (SMON) enq: TX - contention            1            42              4            0     1 20:14:31   20:14:31
              1       3761               1 oracle@<instance_name> (SMON) enq: TX - contention            1             3              3            0     1 20:16:22   20:16:22 <<<<CURRENT_FILE#:3 ==>UNDO
              1       3761               1 oracle@<instance_name> (SMON) enq: TX - contention            1             3              5            0     4 20:22:17   20:25:49 <<<<CURRENT_FILE#:3 ==>UNDO
              1       3761               1 oracle@<instance_name> (SMON) enq: TX - contention            0             3           1920            0   3 20:30:13   20:30:33 <<<<CURRENT_FILE#:3 ==>UNDO
              1       3761               1 oracle@<instance_name> (SMON) enq: TX - contention            0             3          15728            0 23 20:37:58   20:52:39 <<<<CURRENT_FILE#:3 ==>UNDO
              1       3761               1 oracle@<instance_name> (SMON) enq: TX - contention            1          1002              2            0    18 20:53:09   21:04:47

6 rows selected.

SQL>

解决方案
Please avoid to kill large transaction during high workload period.
If has to kill large transaction, shutdown all remote instances can speed up the transaction recovery.
参考:Transaction Recovery Slow And High Row Lock Contention After Killed Large Transaction in RAC (Doc ID 2668617.1)