12c中，每第二天志切换时，会改变alert_sid.log的权限
来源于：
Alert log file's permissions Change with every log switch in 12c (Doc ID 1637367.1)
适用于：
Oracle Database - Enterprise Edition - Version 12.1.0.1 and later
Information in this document applies to any platform.
症状：
When trying to set the group write bit in the OS Permissions for the alert log file it gets reset after every log switch
For example:
[oracle@rhel59 trace]$ pwd
/u01/app/oracle/diag/rdbms/orcl/orcl/trace
[oracle@rhel59 trace]$ ls -lrt alert_orcl.log
-rw-r----- 1 oracle oinstall 357531 May 2 07:30 alert_orcl.log --->此时的权限为640
[oracle@rhel59 trace]$
--修改成666
[oracle@rhel59 trace]$ chmod 666 alert_orcl.log
[oracle@rhel59 trace]$ ls -lrt alert_orcl.log
-rw-rw-rw- 1 oracle oinstall 357988 May 2 07:42 alert_orcl.log--->此时的权限为666
--日志切换
SQL> alter system switch logfile;
System altered.
--再次查看权限
[oracle@rhel59 trace]$ ls -lrt alert_orcl.log
-rw-r--rw- 1 oracle oinstall 358240 May 2 07:50 alert_orcl.log--->此时的权限为646
[oracle@rhel59 trace]$
缘由：
This is an expected behavior, if it is required to have the group write bit set for the alert log, then group write bit needs to be set for in the ADR home directory.
解决方案：
chmod 770 /<SID>/diag/rdbms/<SID>/<SID>
Then restart the database