rac中需要指定thread
添加日志组RAC：
alter database add logfile thread 1 group 1('+DATA/irac/redo01_1.log','+DATA/irac/redo01_2.log') size 1024m;
alter database add logfile thread 1 group 2('+DATA/irac/redo02_1.log','+DATA/irac/redo02_2.log') size 1024m;
单实例：
alter database add logfile group 1('+DATA/irac/redo01_1.log','+DATA/irac/redo01_2.log') size 1024m;
添加日志文件，不需要制定size大小：
alter database add logfile member '/u01/app/oracle/oradata/XXX/redo01_b.log' to group 1;
alter database add logfile member '/u01/app/oracle/oradata/XXX/redo02_b.log' to group 2;
alter database add logfile member '/u01/app/oracle/oradata/XXX/redo03_b.log' to group 3;