SMON后台进程的作用还包括维护MON_MODS$基表，当初始化参数STATISTICS_LEVEL被设置为TYPICAL或ALL时默认会启用Oracle中表监控的特性，Oracle会默认监控表上的自上一次分析以后(Last analyzed)发生的INSERT,UPDATE,DELETE以及表是否被TRUNCATE截断，并将这些操作数量的近似值记录到数据字典基表MON_MODS$中，我们常用的一个DML视图dba_tab_modifications的数据实际来源于另一个数据字典基表MON_MODS_ALL$，SMON定期会将MON_MODS$中符合要求的数据MERGE到MON_MODS_ALL$中。
Rem DML monitoring
create table mon_mods$
(
obj# number, /* object number */
inserts number, /* approx. number of inserts since last analyze */
updates number, /* approx. number of updates since last analyze */
deletes number, /* approx. number of deletes since last analyze */
timestamp date, /* timestamp of last time this row was changed */
flags number, /* flags */
/* 0x01 object has been truncated */
drop_segments number /* number of segemnt in part/subpartition table */
)
storage (initial 200K next 100k maxextents unlimited pctincrease 0)
/
create unique index i_mon_mods$_obj on mon_mods$(obj#)
storage (maxextents unlimited)
/
Rem DML monitoring, has info aggregated to global level for paritioned objects
create table mon_mods_all$
(
obj# number, /* object number */
inserts number, /* approx. number of inserts since last analyze */
updates number, /* approx. number of updates since last analyze */
deletes number, /* approx. number of deletes since last analyze */
timestamp date, /* timestamp of last time this row was changed */
flags number, /* flags */
/* 0x01 object has been truncated */
drop_segments number /* number of segemnt in part/subpartition table */
)
storage (initial 200K next 100k maxextents unlimited pctincrease 0)
/
create unique index i_mon_mods_all$_obj on mon_mods_all$(obj#)
storage (maxextents unlimited)
/
Rem =========================================================================
Rem End Usage monitoring tables
Rem =========================================================================
VIEW DBA_TAB_MODIFICATIONS
select u.name, o.name, null, null,
m.inserts, m.updates, m.deletes, m.timestamp,
decode(bitand(m.flags,1),1,'YES','NO'),
m.drop_segments
from sys.mon_mods_all$ m, sys.obj$ o, sys.tab$ t, sys.user$ u
where o.obj# = m.obj# and o.obj# = t.obj# and o.owner# = u.user#
union all
select u.name, o.name, o.subname, null,
m.inserts, m.updates, m.deletes, m.timestamp,
decode(bitand(m.flags,1),1,'YES','NO'),
m.drop_segments
from sys.mon_mods_all$ m, sys.obj$ o, sys.user$ u
where o.owner# = u.user# and o.obj# = m.obj# and o.type#=19
union all
select u.name, o.name, o2.subname, o.subname,
m.inserts, m.updates, m.deletes, m.timestamp,
decode(bitand(m.flags,1),1,'YES','NO'),
m.drop_segments
from sys.mon_mods_all$ m, sys.obj$ o, sys.tabsubpart$ tsp, sys.obj$ o2,
sys.user$ u
where o.obj# = m.obj# and o.owner# = u.user# and
o.obj# = tsp.obj# and o2.obj# = tsp.pobj#
现象：
SMON后台进程会每15分钟将SGA中的DML统计信息刷新到SYS.MON_MODS$基表中(SMON flush every 15 minutes to SYS.MON_MODS$)，
同时会将SYS.MON_MODS$中符合要求的数据MERGE合并到MON_MODS_ALL$中，并清空原MON_MODS$中的数据。
MON_MODS_ALL$作为dba_tab_modifications视图的数据来源，起到辅助统计信息收集的作用,详见拙作<Does GATHER_STATS_JOB gather all objects’ stats every time?>。
SMON具体将DML统计数据刷新到SYS.MON_MODS$、合并到MON_MODS_ALL$、并清除数据的操作如下：
SQL> select * from v$version;
BANNER
--------------------------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - 64bit Production
PL/SQL Release 11.2.0.2.0 - Production
CORE 11.2.0.2.0 Production
TNS for Linux: Version 11.2.0.2.0 - Production
NLSRTL Version 11.2.0.2.0 - Production
SQL> select * from global_name;
GLOBAL_NAME
--------------------------------------------------------------------------------
www.askmac.cn
/* 填充mon_mods$字典基表 */
lock table sys.mon_mods$ in exclusive mode nowait
insert into sys.mon_mods$
(obj#, inserts, updates, deletes, timestamp, flags, drop_segments)
values
(:1, :2, :3, :4, :5, :6, :7)
update sys.mon_mods$
set inserts = inserts + :ins,
updates = updates + :upd,
deletes = deletes + :del,
flags =
(decode(bitand(flags, :flag), :flag, flags, flags + :flag)),
drop_segments = drop_segments + :dropseg,
timestamp = :time
where obj# = :objn
lock table sys.mon_mods_all$ in exclusive mode
/* 以下merge命令会将mon_mods$中的记录合并到mon_mods_all$,
若有匹配的记录，则在原记录的基础上增加inserts、updates、deletes总数，
否则插入新的记录
*/
merge /*+ dynamic_sampling(mm 4) dynamic_sampling_est_cdn(mm)
dynamic_sampling(m 4) dynamic_sampling_est_cdn(m) */
into sys.mon_mods_all$ mm
using (select m.obj# obj#,
m.inserts inserts,
m.updates updates,
m.deletes deletes,
m.flags flags,
m.timestamp timestamp,
m.drop_segments drop_segments fr om sys.mon_mods$ m,
tab$ t where m.obj# = t.obj#) v
on (mm.ob j# = v.obj#)
when matched then
update
set mm.inserts = mm.inserts + v.inserts,
mm.updates = mm.updates + v.updates,
mm.deletes = mm.deletes + v.deletes,
mm.flags = mm.flags + v.flags - bitand(mm.flags, v.flags) /* bitor(mm.flags,v.flags) */,
mm.timestamp = v.timestamp,
mm.drop_segments = mm.drop_segments + v.drop_segments
when NOT matched then
insert
(obj#, inserts, updates, deletes, timestamp, flags, drop_segments)
values
(v.obj#,
v.inserts,
v.updates,
v.deletes,
sysdate,
v.flags,
v.drop_segments) / all merge /*+ dynamic_sampling(mm 4) dynamic_sampling_est_cdn(mm)
dynamic_sampling(m 4) dynamic_sampling_est_cdn(m) */
into sys.mon_mods_all$ mm using
(select m.obj# obj#,
m.inserts inserts,
m.updates updates,
m.deletes deletes,
m.flags flags,
m.timestamp timestamp,
m.drop_segments drop_segments fr om sys.mon_mods$ m,
tab$ t where m.obj# = t.obj#) v on
(mm.ob j# = v.obj#)
when matched then
update
set mm.inserts = mm.inserts + v.inserts,
mm.updates = mm.updates + v.updates,
mm.deletes = mm.deletes + v.deletes,
mm.flags = mm.flags + v.flags - bitand(mm.flags, v.flags)
/* bitor(mm.flags,v.flags) */,
mm.timestamp = v.timestamp,
mm.drop_segments = mm.drop_segments + v.drop_segments
when NOT matched then
insert
(obj#, inserts, updates, deletes, timestamp, flags, drop_segments)
values
(v.obj#,
v.inserts,
v.updates,
v.deletes,
sysdate,
v.flags,
v.drop_segments)
/* 最后删除sys.mon_mods$上的相关记录 */
delete /*+ dynamic_sampling(m 4) dynamic_sampling_est_cdn(m) */
from sys.mon_mods$ m
where exists (select /*+ unnest */
*
from sys.tab$ t
where t.obj# = m. obj#)
select obj#
from sys.mon_mods$
where obj# not in (select obj# from sys.obj$)
Used to have a FULL TABLE SCAN on obj$ associated with monitoring information
extracted in conjunction with mon_mods$ executed by SMON periodically.
因为当SMON或用户采用”DBMS_STATS.FLUSH_DATABASE_MONITORING_INFO”存储过程将DML数据刷新到mon_mods$或mon_mods_all$中时会要求持有表上的排它锁，所以在RAC环境中可能出现死锁问题。
另外在早期版本中SMON可能因维护监控表而造成shutdown immediate缓慢或系统性能下降的问题，详见:
<Shutdown immediate hangs if table monitoring enabled on [ID 263217.1]>
<Bug 2806297 – SMON can cause bad system performance if TABLE MONITORING enabled on lots of tables [ID 2806297.8]>
SMON维护MON_MODS$时相关的Stack CALL
kglpnal <- kglpin <- kxsGetRuntimeLock <- kksfbc <- kkspsc0 <- kksParseCursor <- opiosq0 <- opiall0 <- opikpr <- opiodr <- PGOSF175_rpidrus <- skgmstack <- rpiswu2 <- kprball <- kprbbnd0 <- kprbbnd <- ksxmfmel <- ksxmfm <- ksxmfchk <- ksxmftim <- ktmmon <- ktmSmonMain <- ksbrdp <- opirip <- opidrv <- sou2o <- opimai_real <- ssthrdmain <- main <- libc_start_main <- start
如何禁止SMON维护MON_MODS$
注意在缺省参数环境中创建的表总是启用table monitoring的:
SQL> select * from v$version; BANNER -------------------------------------------------------------------------------- Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - 64bit Production PL/SQL Release 11.2.0.2.0 - Production CORE 11.2.0.2.0 Production TNS for Linux: Version 11.2.0.2.0 - Production NLSRTL Version 11.2.0.2.0 - Production SQL> select * from v$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> create table maclean1 (t1 int); Table created. /* 在10g以后nomonitoring或monitoring选项不再有效 */ SQL> create table maclean2 (t1 int) nomonitoring; Table created. SQL> select table_name,monitoring from dba_tables where table_name like 'MACLEAN%'; TABLE_NAME MON ------------------------------ --- MACLEAN1 YES MACLEAN2 YES
通常来说我们不需要禁止SMON维护MON_MODS$，除非是在SMON维护过程中遭遇shutdown过慢、性能降低或者异常情况恢复SMON随机terminate实例的问题。
在10g以前可以使用MONITORING和NOMONITORING这2个选项来控制表级别的监控是否被开启，此外我们还可以通过dbms_stats.ALTER_SCHEMA_TAB_MONITORING(‘maclean’,false)存储过程在schema级别的monitoring是否被开启，但是在10g以后这些方法不再有效，MONITORING和NOMONITORING选项被废弃(In 10g the MONITORING and NOMONITORING keywords are deprecated and will be ignored.)，其原有功能被STATISTICS_LEVEL参数所覆盖。
Table-monitoring特性现在完全由STATISTICS_LEVEL参数所控制：
当STATISTICS_LEVEL设置为BASIC时，Table-monitoring将被禁用
当STATISTICS_LEVEL设置为TYPICAL或ALL时,Table-monitoring将启用
换而言之我们可以通过设置STATISTICS_LEVEL为BASIC达到禁止SMON后台进程该种功能的作用，具体修改该参数的命令如下：
show parameter statistics_level alter system set statistics_level = basic;
但是请注意如果你正在使用AMM或ASMM自动内存管理特性的话，那么STATISTICS_LEVEL参数是不能设置为BASIC的，因为Auto-Memory或Auto-Sga特性都依赖于STATISTICS_LEVEL所控制的性能统计信息。若一定要这样做那么首先要diable AMM&ASMM:
#diable 11g AMM ,have to bounce instance #alter system set memory_target =0 scope=spfile; #diable 10g ASMM alter system set sga_target=0; alter system set statistics_level = basic;
