CentOS7上yum提示db错误

Question

从centos6转来7，准备yum update时报错 No Presto metadata available for base

Google 之，大意是我的vps上前端有proxy，在/etc/yum.conf内增加了http_caching=packages，同时尝试yum --enablerepo=base clean metadata ，未果

重启后再次yum,发现报错：

error: rpmdb: BDB0113 Thread/process 3698/140159826708288 failed: BDB1507 Thread             died in Berkeley DB library
error: db5 error(-30973) from dbenv->failchk: BDB0087 DB_RUNRECOVERY: Fatal erro            r, run database recovery
error: cannot open Packages index using db5 -  (-30973)
error: cannot open Packages database in /var/lib/rpm
CRITICAL:yum.main:

WTF.....是什么原因导致的？
感谢