之前我介绍了<Oracle Internal Event:10200 Consistent Read诊断事件>一致性逻辑读诊断事件的用法和trace含义，10201 “consistent read undo application”是另一个十分有用的内部诊断事件，该事件可以用于诊断一致性读取时的UNDO应用情况。
10201 event可以用于探测为了创建CR(consistent read) block块以满足要求的SCN需要应用多少undo，该10201 event还可以配和10200 event使用。利用该10201 event，我们可以验证一些内部问题，例如何时会发生块上的cleanup。
注意启用10201 event可能导致在短期内产生大量的trace文件，所以不要随意在生产系统中使用。
10201 Internal Event主要会被ktrgcm( CR-rollback (ktrgcm() ) 、 ktrrbkblk 、 ktrcrf 这三个Oracle内核函数触发，这三个Internal Function的主要作用：
- ktrgcm – common CR read code
CR Requestor-Side Algorithm
The following statistics are incremented by ktrgcm:
“cleanouts and rollbacks – consistent read” is incremented if UNDO is applied to BUFFER and CLEANOUT is performed.
“rollbacks only – consistent read gets” is incremented if UNDO is applied to BUFFER and no CLEANOUT is performed.
“cleanouts only – consistent read gets” is incremented if no UNDO is applied and CLEANOUT is performed.
“no work – consistent read gets” is incremented if no UNDO is applied and no CLEANOUT is performed.
When UNDO is applied to produce a CR BUFFER, other UNDO blocks should be read.
When CLEANOUT is performed, the TX transaction table must be read.
- ktrrbkblk retrieves previous row version with ktundo,When all rows checked, calls ktrrbkblk to rollback block (calls ktundo) 常见的stack call : ktrviupk kdiulk kcoubk ktundo kturbk ktrrbkblk ktrvfxs qerixFetch qertbFetchByRowID
- ktrcrf (rdbms/kernel/knl/ktr.c kcbchg1 ==> ktrcrf)
SQL> select * from v$version;
BANNER
--------------------------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - 64bit Production
PL/SQL Release 11.2.0.2.0 - Production
CORE 11.2.0.2.0 Production
TNS for Linux: Version 11.2.0.2.0 - Production
NLSRTL Version 11.2.0.2.0 - Production
SQL> select * from global_name;
GLOBAL_NAME
--------------------------------------------------------------------------------
www.askmac.cn & www.askmac.cn
SQL> create table maclean (t1 int);
Table created.
SQL> insert into maclean values(1);
1 row created.
SQL> commit;
Commit complete.
SQL> select current_scn from v$database;
CURRENT_SCN
-----------
1213588
SQL> delete maclean;
1 row deleted.
SQL> commit;
Commit complete.
SQL> alter system flush buffer_cache;
System altered.
SQL> alter system flush buffer_cache;
System altered.
SQL> alter session set events '10201 trace name context forever,level 10';
Session altered.
SQL> select * from maclean as of scn 1213588;
T1
----------
1
trace content=====================================
Applying CR undo to block 0 : 408a81 itl entry 02:
xid: 0x0009.00b.0000017d uba: 0x00c00212.0092.2d
flg: --U- lkc: 1 fsc: 0x0007.0012849c
CRS upd rd env [0x2ac2ec7b5660]: (scn: 0x0000.00128494 xid: 0x0000.000.00000000
uba: 0x00000000.0000.00 statement num=0 parent xid: 0x0000.000.00000000 st-scn: 0x
0000.00000000 hi-scn: 0x0000.00000000 ma-scn: 0x0000.00000000 flg: 0x00000800)
undo env [0x7fffe8b74d10]: (
scn: 0x0000.0012849b xid: 0x0009.00b.0000017d uba: 0x00c00212.0092.2d statement num=151548811
parent xid: 0x0000.000.00000000 st-scn: 0x0000.00000000 hi-scn: 0x9
f58.00000000 ma-scn: 0x2ac2.ec7b9c88 flg: 0x00002ac2)
CRS upd (before): 0x69bdfe68 cr-scn: 0x0000.00128494 xid: 0x0000.000.00000000
uba: 0x00000000.0000.00 cl-scn: 0x0000.001287c1 sfl: 0
CRS upd (after) : 0x69bdfe68 cr-scn: 0x0000.00128494 xid: 0x0009.00b.0000017d
uba: 0x00c00212.0092.2d cl-scn: 0x0000.001287c1 sfl: 0
以上trace中各代码的含义如下：
Applying CR undo to block 0 : 408a81 itl entry 02:
这里的0是 tablespace number, 408a81 是 DBA
而 itl entry 02 是被回滚的事务槽记录
CRS upd rd env [0x2ac2ec7b5660]: (scn: 0x0000.00128494 …..undo env [0x7fffe8b74d10]: (
以上为当前读取的环境信息，包括env_scn等
CRS upd (before): 0x69bdfe68 cr-scn: 0x0000.00128494
xid: 0x0000.000.00000000 uba: 0x00000000.0000.00 cl-scn: 0x0000.001287c1 sfl: 0
CRS upd (before)为回滚完成前的Buffer descriptor
CRS upd (after) : 0x69bdfe68 cr-scn: 0x0000.00128494 CR-SCN为1213588 如查询语句所要求的
xid: 0x0009.00b.0000017d uba: 0x00c00212.0092.2d cl-scn: 0x0000.001287c1 sfl: 0
CRS upd (after) 为回滚完成后的Buffer descriptor
