备库数据文件是不允许写数据，切换日志是需要写数据到数据文件中的。为什么rman备份可以，是因为rman备份写的是控制文件~

1、备库演示，会报错，数据库为打开。

SQL> archive log list;
Database log mode          Archive Mode
Automatic archival         Enabled
Archive destination        /arch
Oldest online log sequence     4132
Next log sequence to archive   0
Current log sequence           4134
SQL> alter system archive log current;
alter system archive log current
*
ERROR at line 1:
ORA-01109: database not open

SQL>