Starting backup at 01-SEP-11
using channel ORA_DISK_1
RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-03002: failure of backup command at 09/01/2011 00:30:19
RMAN-20021: database not set
RMAN-06031: could not translate database keyword
原因是在另一个窗口将数据库由mount改到open操作引起的：
SQL> ALTER DATABASE OPEN;
此时，RMAN未接获数据库打开的通知，退出，再从新连入即可解决。
（如果不行尝试设置nls_lang，其实应该不用的）