系统环境：
操作系统： AIX 5300-08
Oracle： Oracle 10.2.0.1.0
错误现象：
RMAN> restore datafile 4;
Starting restore at 20-MAY-14
allocated channel: ORA_DISK_1
channel ORA_DISK_1: sid=139 devtype=DISK
channel ORA_DISK_1: starting datafile backupset restore
channel ORA_DISK_1: specifying datafile(s) to restore from backup set
restoring datafile 00004 to /u01/app/oracle/oradata/prod/users01.dbf
channel ORA_DISK_1: reading from backup piece /backup/inc0_PROD_2qp8omr6_1_1
RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-03002: failure of restore command at 05/20/2014 12:19:41
ORA-19870: error reading backup piece /backup/inc0_PROD_2qp8omr6_1_1
ORA-19573: cannot obtain exclusive enqueue for datafile 4
一直以为是备份有问题，没有找到备份的数据文件！
原来是DataBase 在open下，忘了将datafile offline，造成此低级错误；将数据文件offline 后，问题解决！
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