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why doesn't alter system set events set the events or tracing immediately
why doesn't alter system set events set the events or tracing immediately?
今天看到Tanel的blog，说明了此问题：
我们通过alter system 设置events和parameter是不同的，
对于设置parameter使用alter system的影响如下：
1）对当前的session起作用
2）对新用户登录数据库的用户起作用
3）对已经登录到数据库的其他session起作用
对于使用alter system 设置events影响如下：
1）对当前session起作用
2）对新登录到系统中的用户起作用，但不对其他的session起作用。
1、对于alter system set parameters验证如下：
eg：
目前系统只有三个session登录：
[sql] SQL> select saddr,sid,serial#,username from v$session where username is not null; SADDR SID SERIAL# USERNAME ---------------- ---------- ---------- ------------------------------ 000000007CFED5D8 33 103 SYS 000000007CFDE178 38 129 SCOTT 000000007CFD4ED8 41 65 RHYS SQL> 首先修改log_checkpoints_to_alert参数为true，当前用户为sys；[sql] SQL> show parameter log_checkpoint NAME TYPE VALUE ------------------------------------ ----------- ------------------------------ log_checkpoint_interval integer 0 log_checkpoint_timeout integer 1800 log_checkpoints_to_alert boolean FALSE SQL> alter system set log_checkpoints_to_alert=true; System altered. SQL> SQL> show user USER is "SYS" SQL> 在rhys账户查看该parameter值：[sql] SQL> show user USER is "RHYS" SQL> select * from v$mystat where rownum<3; SID STATISTIC# VALUE ---------- ---------- ---------- 41 0 0 41 1 12 SQL> show parameter log_checkpoints NAME TYPE VALUE ------------------------------------ ----------- ------------------------------ log_checkpoints_to_alert boolean TRUE SQL>
在scott账户查看该parameter值：
[sql] SQL> show user USER is "SCOTT" SQL> select * from v$mystat where rownum<3; SID STATISTIC# VALUE ---------- ---------- ---------- 38 0 0 38 1 12 SQL> show parameter log_checkpoints NAME TYPE VALUE ------------------------------------ ----------- ------------------------------ log_checkpoints_to_alert boolean TRUE SQL>
新登录一个用户Amy；
[sql] SQL> create user amy identified by rhys default tablespace rhys; User created. SQL> grant dba to amy; Grant succeeded. SQL> select saddr,sid,serial#,username from v$session where username is not null; SADDR SID SERIAL# USERNAME ---------------- ---------- ---------- ------------------------------ 000000007C8836E0 1 83 AMY 000000007CFED5D8 33 103 SYS 000000007CFDE178 38 137 SCOTT 000000007CFD4ED8 41 65 RHYS SQL> SQL> show user USER is "AMY" SQL> select * from v$mystat where rownum<3; SID STATISTIC# VALUE ---------- ---------- ---------- 1 0 0 1 1 12 SQL> show parameter log_checkpoints_to_alert NAME TYPE VALUE ------------------------------------ ----------- ------------------------------ log_checkpoints_to_alert boolean TRUE SQL>
2、对于对于alter system set events验证如下：
[sql] SQL> select saddr,sid,serial#,username from v$session where username is not null; SADDR SID SERIAL# USERNAME ---------------- ---------- ---------- ------------------------------ 000000007CFED5D8 33 103 SYS 000000007CFDE178 38 137 SCOTT 000000007CFD4ED8 41 65 RHYS SQL> show user USER is "SYS" SQL> alter system set events '10046 trace name context forever,level 12'; System altered. SQL> SQL> select saddr,sid,s.serial#,s.username,spid from v$session s,v$process p where s.paddr=p.addr and s.username is not null; SADDR SID SERIAL# USERNAME SPID ---------------- ---------- ---------- ------------------------------ ------------------------ 000000007CFDE178 38 137 SCOTT 2176 000000007CFED5D8 33 103 SYS 1981 000000007CFD4ED8 41 65 RHYS 2092 SQL> SQL> ! [oracle@oracle-one ~]$ cd /opt/app/oracle/diag/rdbms/rhys/RHYS/trace/ [oracle@oracle-one trace]$ ls -l *1981* -rw-r-----. 1 oracle oinstall 23897 Oct 9 09:58 RHYS_ora_1981.trc -rw-r-----. 1 oracle oinstall 390 Oct 9 09:58 RHYS_ora_1981.trm [oracle@oracle-one trace]$
可以看到sys用户已经存在trace文件了。
然后在rhys用户下查看一下信息：
[sql] SQL> select * from v$mystat where rownum<3; SID STATISTIC# VALUE ---------- ---------- ---------- 41 0 0 41 1 19 SQL> SQL> select table_name from user_tables; TABLE_NAME ------------------------------ DEPT BONUS AMY A B C T SYS_TEMP_FBT SALGRADE EMP 10 rows selected. SQL> ! [oracle@oracle-one ~]$ cd /opt/app/oracle/diag/rdbms/rhys/RHYS/trace/ [oracle@oracle-one trace]$ ls -ltr *2092* -rw-r-----. 1 oracle oinstall 1211 Oct 9 10:02 RHYS_ora_2092.trm -rw-r-----. 1 oracle oinstall 164508 Oct 9 10:02 RHYS_ora_2092.trc [oracle@oracle-one trace]$
同样也存在trace文件了。
那么对于新增的用户呢？我们使用Amy账户登录。
[sql] SQL> show user USER is "AMY" SQL> select saddr,sid,s.serial#,s.username,spid from v$session s,v$process p where s.paddr=p.addr and s.username is not null; SADDR SID SERIAL# USERNAME SPID ---------------- ---------- ---------- ------------------------------ ------------------------ 000000007CFDE178 38 137 SCOTT 2176 000000007CFED5D8 33 103 SYS 1981 000000007CFD4ED8 41 65 RHYS 2092 000000007CFC2998 47 137 AMY 5500 SQL> select * from v$mystat where rownum<3; SID STATISTIC# VALUE ---------- ---------- ---------- 47 0 0 47 1 18 SQL> ! [oracle@oracle-one ~]$ cd /opt/app/oracle/diag/rdbms/rhys/RHYS/trace/ [oracle@oracle-one trace]$ ls -l *5500* -rw-r-----. 1 oracle oinstall 184626 Oct 9 10:25 RHYS_ora_5500.trc -rw-r-----. 1 oracle oinstall 1095 Oct 9 10:25 RHYS_ora_5500.trm [oracle@oracle-one trace]$
发现也是可以跟踪到trace。
可知在11g中，alter system set events也是对已经登录的会话生效的。但是在10G确不是。
note：
如果在不知情的情况下设置了跟踪事件，想想那是多么可怕的事情。可能导致磁盘占满，系统宕机等事故。
如下是转自tanel的一句话：
This means that the existing, already logged in sessions, will not pick up any of the events set via ALTER SYSTEM!
This hopefully explains why sometimes the debug events don’t seem to work. But more importantly, this also means that when you disable an event (by setting it to “OFF” or to level 0) with ALTER SYSTEM, it does not affect the existing sessions who have this event enabled! So, you think you’re turning the tracing off for all sessions and go home, but really some sessions keep on tracing – until the filesystem is full (and you’ll get a phone call at 3am).
So, to be safe, you should use DBMS_MONITOR for your SQL Tracing needs, it doesn’t have the abovementioned problems. For other events you should use DBMS_SYSTEM.SET_EV/READ_EV (or ORADEBUG EVENT/SESSION_EVENT & EVENTS/EVENTDUMP) together with ALTER SYSTEM for making sure you actually do enable/disable the events for all existing sessions too. Or better yet, stay away from undocumented events ;-)
If you wonder what/where is the “system event array”, it’s just a memory location in shared pool. It doesn’t seem to be explicitly visible in V$SGASTAT in Oracle 10g, but in 11.2.0.3 you get this:
No system-wide events set:
SQL> @sgastat eventPOOL NAME BYTES------------ -------------------------- ----------shared pool DBWR event stats array 216shared pool KSQ event description 8460shared pool Wait event pointers 192shared pool dbgdInitEventGrp: eventGr 136shared pool event classes 1552shared pool event descriptor table 32360shared pool event list array to post 36shared pool event list to post commit 108shared pool event statistics per sess 2840096shared pool event statistics ptr arra 992shared pool event-class map 4608shared pool ksws service events 57260shared pool latch wait-event table 2212shared pool standby event stats 1216shared pool sys event stats 539136shared pool sys event stats for Other 32256shared pool trace events array 7200017 rows selected.Let’s set a system-wide event:SQL> ALTER SYSTEM SET events = '942 TRACE NAME ERRORSTACK LEVEL 3'; System altered.And check V$SGASTAT again:SQL> @sgastat eventPOOL NAME BYTES------------ -------------------------- ----------shared pool DBWR event stats array 216shared pool KSQ event description 8460shared pool Wait event pointers 192shared pool dbgdInitEventG 4740shared pool dbgdInitEventGrp: eventGr 340shared pool dbgdInitEventGrp: subHeap 80shared pool event classes 1552shared pool event descriptor table 32360shared pool event list array to post 36shared pool event list to post commit 108shared pool event statistics per sess 2840096shared pool event statistics ptr arra 992shared pool event-class map 4608shared pool ksws service events 57260shared pool latch wait-event table 2212shared pool standby event stats 1216shared pool sys event stats 539136shared pool sys event stats for Other 32256shared pool trace events array 7200019 rows selected.
So, the “system event array” lives in shared pool, as a few memory allocations with name like “dbgdInitEventG%”. Note that this naming was different in 10g, as the dbgd module showed up in Oracle 11g, when Oracle guys re-engineered the whole diagnostics event infrastructure, making it much more powerful, for example allowing you to enable dumps and traces only for a specific SQL_ID.
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