SMON后台进程的作用还包括维护SMON_SCN_TIME基表。
SMON_SCN_TIME基表用于记录过去时间段中SCN(system change number)与具体的时间戳(timestamp)之间的映射关系，因为是采样记录这种映射关系，所以SMON_SCN_TIME可以较为较为粗糙地(不精确地)定位某个SCN的时间信息。实际的SMON_SCN_TIME是一张cluster table簇表。
SMON_SCN_TIME时间映射表最大的用途是为闪回类型的查询(flashback type queries)提供一种将时间映射为SCN的途径(The SMON time mapping is mainly for flashback type queries to map a time to an SCN)。
Metalink文档<Error ORA-01466 while executing a flashback query. [ID 281510.1]>介绍了SMON更新SMON_SCN_TIME的规律：
- 在版本10g中SMON_SCN_TIME每6秒钟被更新一次(In Oracle Database 10g, smon_scn_time is updated every 6 seconds hence that is the minimum time that the flashback query time needs to be behind the timestamp of the first change to the table.)
- 在版本9.2中SMON_SCN_TIME每5分钟被更新一次(In Oracle Database 9.2, smon_scn_time is updated every 5 minutes hence the required delay between the flashback time and table properties change is at least 5 minutes.)
另外从10g开始SMON也会清理SMON_SCN_TIME中的记录了，SMON后台进程会每5分钟被唤醒一次，检查SMON_SCN_TIME在磁盘上的映射记录总数，若总数超过144000条，则会使用以下语句删除最老的一条记录(time_mp最小):
delete from smon_scn_time where thread = 0 and time_mp = (select min(time_mp) from smon_scn_time where thread = 0)
若仅仅删除一条记录不足以获得足够的空间，那么SMON会反复多次执行以上DELETE语句。
触发场景
虽然Metalink文档<Error ORA-01466 while executing a flashback query. [ID 281510.1]>指出了在10g中SMON会以每6秒一次的频率更新SMON_SCN_TIME基表，但是实际观测可以发现更新频率与SCN的增长速率相关，在较为繁忙的实例中SCN的上升极快时SMON可能会以6秒一次的最短间隔频率更新 ， 但是在空闲的实例中SCN增长较慢，则仍会以每5或10分钟一次频率更新，例如：
[oracle@vrh8 ~]$ ps -ef|grep smon|grep -v grep oracle 3484 1 0 Nov12 ? 00:00:02 ora_smon_G10R21 SQL> select * from v$version; BANNER ---------------------------------------------------------------- Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - 64bi PL/SQL Release 10.2.0.1.0 - Production CORE 10.2.0.1.0 Production TNS for Linux: Version 10.2.0.1.0 - Production NLSRTL Version 10.2.0.1.0 - Production SQL> select * from global_name; GLOBAL_NAME -------------------------------------------------------------------------------- www.askmaclean.com & www.askmaclean.com SQL> oradebug setospid 3484; Oracle pid: 8, Unix process pid: 3484, image: oracle@vrh8.oracle.com (SMON) SQL> oradebug event 10500 trace name context forever,level 10 : 10046 trace name context forever,level 12; Statement processed. SQL> SQL> oradebug tracefile_name; /s01/admin/G10R21/bdump/g10r21_smon_3484.trc /* 等待一定时间 */
找出SMON trace文件中insert数据到SMON_SCN_TIME的记录:
grep -A20 "insert into smon_scn_time" /s01/admin/G10R21/bdump/g10r21_smon_3484.trc insert into smon_scn_time (thread, time_mp, time_dp, scn, scn_wrp, scn_bas, num_mappings, tim_scn_map) values (0, :1, :2, :3, :4, :5, :6, :7) END OF STMT PARSE #4:c=0,e=43,p=0,cr=0,cu=0,mis=0,r=0,dep=1,og=4,tim=1290280848899596 BINDS #4: kkscoacd Bind#0 oacdty=02 mxl=22(22) mxlc=00 mal=00 scl=00 pre=00 oacflg=00 fl2=0001 frm=00 csi=00 siz=24 off=0 kxsbbbfp=7fb29844edb8 bln=22 avl=06 flg=05 value=767145793 Bind#1 oacdty=12 mxl=07(07) mxlc=00 mal=00 scl=00 pre=00 oacflg=10 fl2=0001 frm=00 csi=00 siz=8 off=0 kxsbbbfp=7fff023ae780 bln=07 avl=07 flg=09 value="11/14/2011 0:3:13" Bind#2 oacdty=02 mxl=22(04) mxlc=00 mal=00 scl=00 pre=00 oacflg=10 fl2=0001 frm=00 csi=00 siz=24 off=0 kxsbbbfp=7fff023ae70c bln=22 avl=04 flg=09 value=954389 Bind#3 -- insert into smon_scn_time (thread, time_mp, time_dp, scn, scn_wrp, scn_bas, num_mappings, tim_scn_map) values (0, :1, :2, :3, :4, :5, :6, :7) END OF STMT PARSE #1:c=0,e=21,p=0,cr=0,cu=0,mis=0,r=0,dep=1,og=4,tim=1290281434933390 BINDS #1: kkscoacd Bind#0 oacdty=02 mxl=22(22) mxlc=00 mal=00 scl=00 pre=00 oacflg=00 fl2=0001 frm=00 csi=00 siz=24 off=0 kxsbbbfp=7fb29844edb8 bln=22 avl=06 flg=05 value=767146393 Bind#1 oacdty=12 mxl=07(07) mxlc=00 mal=00 scl=00 pre=00 oacflg=10 fl2=0001 frm=00 csi=00 siz=8 off=0 kxsbbbfp=7fff023ae780 bln=07 avl=07 flg=09 value="11/14/2011 0:13:13" Bind#2 oacdty=02 mxl=22(04) mxlc=00 mal=00 scl=00 pre=00 oacflg=10 fl2=0001 frm=00 csi=00 siz=24 off=0 kxsbbbfp=7fff023ae70c bln=22 avl=04 flg=09 value=954720 Bind#3 -- insert into smon_scn_time (thread, time_mp, time_dp, scn, scn_wrp, scn_bas, num_mappings, tim_scn_map) values (0, :1, :2, :3, :4, :5, :6, :7) END OF STMT PARSE #3:c=0,e=20,p=0,cr=0,cu=0,mis=0,r=0,dep=1,og=4,tim=1290281727955249 BINDS #3: kkscoacd Bind#0 oacdty=02 mxl=22(22) mxlc=00 mal=00 scl=00 pre=00 oacflg=00 fl2=0001 frm=00 csi=00 siz=24 off=0 kxsbbbfp=7fb29844e960 bln=22 avl=06 flg=05 value=767146993 Bind#1 oacdty=12 mxl=07(07) mxlc=00 mal=00 scl=00 pre=00 oacflg=10 fl2=0001 frm=00 csi=00 siz=8 off=0 kxsbbbfp=7fff023ae780 bln=07 avl=07 flg=09 value="11/14/2011 0:23:13" Bind#2 oacdty=02 mxl=22(04) mxlc=00 mal=00 scl=00 pre=00 oacflg=10 fl2=0001 frm=00 csi=00 siz=24 off=0 kxsbbbfp=7fff023ae70c bln=22 avl=04 flg=09 value=954926 Bind#3 insert into smon_scn_time (thread, time_mp, time_dp, scn, scn_wrp, scn_bas, num_mappings, tim_scn_map) values (0, :1, :2, :3, :4, :5, :6, :7) END OF STMT PARSE #4:c=0,e=30,p=0,cr=0,cu=0,mis=0,r=0,dep=1,og=4,tim=1290282313990553 BINDS #4: kkscoacd Bind#0 oacdty=02 mxl=22(22) mxlc=00 mal=00 scl=00 pre=00 oacflg=00 fl2=0001 frm=00 csi=00 siz=24 off=0 kxsbbbfp=7fb29844edb8 bln=22 avl=06 flg=05 value=767147294 Bind#1 oacdty=12 mxl=07(07) mxlc=00 mal=00 scl=00 pre=00 oacflg=10 fl2=0001 frm=00 csi=00 siz=8 off=0 kxsbbbfp=7fff023ae780 bln=07 avl=07 flg=09 value="11/14/2011 0:28:14" Bind#2 oacdty=02 mxl=22(04) mxlc=00 mal=00 scl=00 pre=00 oacflg=10 fl2=0001 frm=00 csi=00 siz=24 off=0 kxsbbbfp=7fff023ae70c bln=22 avl=04 flg=09 value=955036 Bind#3
可以通过以上INSERT语句的TIME_DP绑定变量值中发现其更新SMON_SCN_TIME的时间规律，一般为5或10分钟一次。这说明SMON_SCN_TIME的更细频率与数据库实例的负载有关，其最短的间隔是每6秒一次，最长的间隔为10分钟一次。
由于SMON_SCN_TIME的更新频率问题可能引起ORA-01466错误，详见:
Error ORA-01466 while executing a flashback query. [ID 281510.1]
由于SMON_SCN_TIME的数据不一致可能引起ORA-00600[6711]或频繁地执行”delete from smon_scn_time”删除语句，详见：
ORA-00600[6711]错误一例
High Executions Of Statement “delete from smon_scn_time…” [ID 375401.1]
SMON维护SMON_SCN_TIME时相关的Stack CALL，ktf_scn_time是更新SMON_SCN_TIME的主要函数:
ksedst ksedmp ssexhd kghlkremf kghalo kghgex kghalf kksLoadChild kxsGetRuntimeLock kksfbc
kkspsc0 kksParseCursor opiosq0 opiall0 opikpr opiodr rpidrus skgmstack rpidru rpiswu2 kprball
ktf_scn_time
ktmmon ktmSmonMain ksbrdp opirip opidrv sou2o opimai_real main main_opd_entry
SMON 还可能使用以下SQL语句维护SMON_SCN_TIME字典基表：
select smontabv.cnt,
smontab.time_mp,
smontab.scn,
smontab.num_mappings,
smontab.tim_scn_map,
smontab.orig_thread
from smon_scn_time smontab,
(select max(scn) scnmax,
count(*) + sum(NVL2(TIM_SCN_MAP, NUM_MAPPINGS, 0)) cnt
from smon_scn_time
where thread = 0) smontabv
where smontab.scn = smontabv.scnmax
and thread = 0
insert into smon_scn_time
(thread,
time_mp,
time_dp,
scn,
scn_wrp,
scn_bas,
num_mappings,
tim_scn_map)
values
(0, :1, :2, :3, :4, :5, :6, :7)
update smon_scn_time
set orig_thread = 0,
time_mp = :1,
time_dp = :2,
scn = :3,
scn_wrp = :4,
scn_bas = :5,
num_mappings = :6,
tim_scn_map = :7
where thread = 0
and scn = (select min(scn) from smon_scn_time where thread = 0)
delete from smon_scn_time
where thread = 0
and scn = (select min(scn) from smon_scn_time where thread = 0)
如何禁止SMON更新SMON_SCN_TIME基表
可以通过设置诊断事件event=’12500 trace name context forever, level 10’来禁止SMON更新SMON_SCN_TIME基表(Setting the 12500 event at system level should stop SMON from updating the SMON_SCN_TIME table.):
SQL> alter system set events '12500 trace name context forever, level 10'; System altered.
一般我们不推荐禁止SMON更新SMON_SCN_TIME基表，因为这样会影响flashback Query闪回查询的正常使用，但是在某些异常恢复的场景中SMON_SCN_TIME数据讹误可能导致实例的Crash，那么可以利用以上12500事件做到不触发SMON_SCN_TIME被更新。
如何手动清除SMON_SCN_TIME的数据
因为SMON_SCN_TIME不是bootstrap自举核心对象，所以我们可以手动更新该表上的数据、及重建其索引。
如我在<ORA-00600[6711]错误一例>中介绍了因为SMON_SCN_TIME与其索引的数据不一致时，可以通过重建索引来解决问题：
connect / as sysdba drop index smon_scn_time_scn_idx; drop index smon_scn_time_tim_idx; create unique index smon_scn_time_scn_idx on smon_scn_time(scn); create unique index smon_scn_time_tim_idx on smon_scn_time(time_mp); analyze table smon_scn_time validate structure cascade;
可以在设置了12500事件后手动删除SMON_SCN_TIME上的记录，重启实例后SMON会继续正常更新SMON_SCN_TIME。除非是因为SMON_SCN_TIME表上的记录与索引smon_scn_time_tim_idx或smon_scn_time_scn_idx上的不一致造成DELETE语句无法有效删除该表上的记录:文档<LOCK ON SYS.SMON_SCN_TIME [ID 747745.1]>说明了该问题，否则我们没有必要手动去清除SMON_SCN_TIME上的数据。
具体方法如下：
SQL> conn / as sysdba /* Set the event at system level */ SQL> alter system set events '12500 trace name context forever, level 10'; /* Delete the records from SMON_SCN_TIME */ SQL> delete from smon_scn_time; SQL> commit; SQL> alter system set events '12500 trace name context off'; 完成以上步骤后重启实例restart instance shutdown immediate; startup;
SMON_SCN_TIME表存放的是SCN和Time之前的映射关系。 该表由SMON 进程负责维护。
SQL> desc smon_scn_time
Name Null? Type
------------------------------------------------- ----------------------------
THREAD NUMBER
TIME_MP NUMBER
TIME_DP DATE
SCN_WRP NUMBER
SCN_BAS NUMBER
NUM_MAPPINGS NUMBER
TIM_SCN_MAP RAW(1200)
SCN NUMBER
ORIG_THREAD NUMBER
SQL> alter session set nls_date_format='yyyy-mm-ddhh24:mi:ss';
Session altered.
SQL> select time_dp,scn from smon_scn_time where rownum<5;
TIME_DP SCN
------------------- ----------
2013-03-15 10:31:04 2092348
2013-03-15 10:35:49 2092452
2013-03-15 10:41:00 2092581
2013-03-15 10:45:46 2092682
在Oracle 11g中，该表的创建SQL在$ORACLE_HOME/rdbms/admin/dtxnspc.bsq 文件中。
create table smon_scn_time (
thread number, /* thread, compatibility */
time_mp number, /* time this recent scn represents */
time_dp date, /* time as date, compatibility */
scn_wrpnumber, /*scn.wrp, compatibility */
scn_bas number, /* scn.bas, compatibility */
num_mappings number,
tim_scn_map raw(1200),
scnnumber default 0, /* scn*/
orig_thread number default 0 /* for downgrade */
) cluster smon_scn_to_time_aux (thread)
/
create unique index smon_scn_time_tim_idxon smon_scn_time(time_mp)
tablespace SYSAUX
/
create unique index smon_scn_time_scn_idxon smon_scn_time(scn)
tablespace SYSAUX
/
我们可以直接delete掉SMON_SCN_TIME表中的记录：
SQL> delete from smon_scn_time;
2120 rows deleted.
SQL> commit;
Commit complete.
SQL> select count(1) from smon_scn_time;
COUNT(1)
----------
0
二．SMON_SCN_TIME表记录保存策略说明
2.1 Oracle 9i
根据MOS文档的说明：
How To Map SCN To Timestamp Before 10g? [ID365536.1]
SYS.SMON_SCN_TIMEwill have a maximum of 1440 rows and each record will be for a 5 minute period.Oracle maintains this information for a maximum of 5 days after which therecords will be recycled.
This means thatdata is stored 12 times per hour * 24 hours * 5 days = 1440 rows.
在Oracle 9i版本中，SMON_SCN_TIME 表中最多存放1440条记录。 SMON 进程每隔5分钟生成一次SCN和TIME 之前的映射，并更新到SMON_SCN_TIME表。该表的维护周期是5天。
因此该表最多存放的记录是：12*24*5=1440条记录。
超过1440条的记录在下次循环中会被删除。
2.2 Oracle 10g以后的版本
在oracle 10g以后的版本，SMON_SCN_TIME表的维护策略发生了变化。
根据MOS文档的说明：
High Executions Of Statement "deletefrom smon_scn_time..." [ID 375401.1]
The deletestatement deletes the oldest rows from smon_scn_time to clear space for newrows. SMON wakes up every 5 minutes and checks how many on-disk mappingswe have--the max is 144000.
--SMON进程每个5分钟唤醒一次来更新SCN和TIME之间的映射关系，并且检查SMON_SCN_TIME表中的记录数，该表的记录数最大是144000条。
The new mappingsare then added for the last period (since SMON last updated), and if this isover 144000, SMON will then issue the delete statement:
delete fromsmon_scn_time where thread=0 and time_mp = (select min(time_mp) fromsmon_scn_time where thread=0)
--SMON进程会把最新的SCN_TIME映射关系写入SMON_SCN_TIME表，如果该表的记录数超过144000条，那么就会执行上面的delete操作，删除最早的记录。
There will be anexecution of this each time SMON wakes to update smon_scn_time, and if onedeletion does not free enough mappings, then there will be multiple executions.
--SMON进程每次被唤醒都会更新SMON_SCN_TIME表，如果一次delete操作不能释放足够的空间映射空间，就会执行多次delete操作。
三．禁用SMON 进程对SMON_SCN_TIME 表的更新
可以设置12500事件停止SMON进程对SMON_SCN_TIME。
具体操作如下：
SQL> select count(1) from smon_scn_time;
COUNT(1)
----------
2115
SQL> alter system set events '12500trace name context forever, level 10';
System altered.
SQL> select sysdate from dual;
SYSDATE
-------------------
2013-03-20 13:06:15
SQL> select count(1) from smon_scn_time;
COUNT(1)
----------
2115
SQL> alter system set events '12500 tracename context off';
System altered.
SQL> select sysdate from dual;
SYSDATE
-------------------
2013-03-20 13:19:58
SQL> select count(1) from smon_scn_time;
COUNT(1)
----------
2119
四．SMON_SCN_TIME 表相关的2个案例
4.1 Oracle 9i SMON_SCN_TIME 表被锁
LOCK ON SYS.SMON_SCN_TIME [ID 747745.1]
4.1.1 现象
Oracle 9i，SYS.SMON_SCN_TIME 被 SMON 进程已排它锁占用，并且锁不能释放，导致数据库出现性能问题，并且SMON_SCN_TIME表中有大量的记录。
SQL> selectcount(*) from sys.smon_scn_time;
COUNT(*) ---------- 137545 1 row selected.
--正常情况下，9i最多只能保存1440条记录。
SQL> select object_id from dba_objectswhere object_name = 'SMON_SCN_TIME'; OBJECT_ID ---------- 575 1 row selected.
SQL> select * fromv$locked_object where object_id = 575;
XIDUSNXIDSLOT XIDSQN OBJECT_ID SESSION_ID ---------- ---------- ---------- ---------- ---------- ORACLE_USERNAME OS_USER_NAME PROCESS ------------------------------ ------------------------------ ------------ LOCKED_MODE ----------- 5 5 1494 575 164 dbadmin 4444350 3 <=Locked in row exclusive mode
4.1.2 处理方法
设置12500事件，停止SMON 进程更新SMON_SCN_TIME表，然后手工删除表中的记录。
SQL> alter system set events '12500 tracename context forever, level 10';
SQL> delete from smon_scn_time;
SQL> commit;
SQL> alter system set events '12500 tracename context off';
Now restart the instance.
4.2 Oracle 10g SMON_SCN_TIME 表频繁的被delete
High Executions Of Statement "deletefrom smon_scn_time..." [ID 375401.1]
4.2.1 现象
AWR报告显示smon_scn_time的删除操作频繁的被执行。
delete fromsmon_scn_time where thread=0 and time_mp = (select min(time_mp) fromsmon_scn_time where thread=0);
导致这种现象的原因是SMON_SCN_TIME表和表上的索引不一致。需要重建SMON_SCN_TIME上的索引。
SQL> analyze table smon_scn_timevalidate structure cascade; analyze table smon_scn_time validate structure cascade * ERROR at line 1 : ORA-01499: table/Index Cross Reference Failure - see trace file
4.2.2 处理方法
connect / as sysdba drop index smon_scn_time_scn_idx; drop index smon_scn_time_tim_idx; create unique index smon_scn_time_scn_idx on smon_scn_time(scn); create unique index smon_scn_time_tim_idx on smon_scn_time(time_mp); analyze table smon_scn_time validate structure cascade;