今天用yum 安装ntp 的过程中用了 Ctrl+ z， 然后yum 再也不能使用了：
error: rpmdb: BDB0113 Thread/process 6589/140601939367744 failed: BDB1507 Thread died in Berkeley DB library
error: db5 error(-30973) from dbenv->failchk: BDB0087 DB_RUNRECOVERY: Fatal error, run database recovery
error: cannot open Packages index using db5 - (-30973)
error: cannot open Packages database in /var/lib/rpm
CRITICAL:yum.main:
Error: rpmdb open failed
解决方法：
rpm --rebuilddb
yum clean all