自从CentOS7.5更新以后，CentOS7.4以及CentOS7.5都出现存储库无法使用的问题，具体如下:
执行以下命令安装OpenStack存储库
[root@localhost ~]# yum install centos-release-openstack-queens导致CentOS源无法使用，报错信息如下：
[root@localhost ~]# yum makecache
Loaded plugins: fastestmirror
base | 3.6 kB 00:00:00
centos-ceph-luminous | 2.9 kB 00:00:00
centos-openstack-queens | 2.9 kB 00:00:00
http://mirror.centos.org/%24contentdir/7/virt/x86_64/kvm-common/repodata/repomd.xml: [Errno 14] HTTP Error 404 - Not Found
Trying other mirror.
To address this issue please refer to the below wiki article
https://wiki.centos.org/yum-errors
If above article doesn't help to resolve this issue please use https://bugs.centos.org/.
One of the configured repositories failed (CentOS-7 - QEMU EV),
and yum doesn't have enough cached data to continue. At this point the only
safe thing yum can do is fail. There are a few ways to work "fix" this:
1. Contact the upstream for the repository and get them to fix the problem.
2. Reconfigure the baseurl/etc. for the repository, to point to a working
upstream. This is most often useful if you are using a newer
distribution release than is supported by the repository (and the
packages for the previous distribution release still work).
3. Run the command with the repository temporarily disabled
yum --disablerepo=centos-qemu-ev ...
4. Disable the repository permanently, so yum won't use it by default. Yum
will then just ignore the repository until you permanently enable it
again or use --enablerepo for temporary usage:
yum-config-manager --disable centos-qemu-ev
or
subscription-manager repos --disable=centos-qemu-ev
5. Configure the failing repository to be skipped, if it is unavailable.
Note that yum will try to contact the repo. when it runs most commands,
so will have to try and fail each time (and thus. yum will be be much
slower). If it is a very temporary problem though, this is often a nice
compromise:
yum-config-manager --save --setopt=centos-qemu-ev.skip_if_unavailable=true
failure: repodata/repomd.xml from centos-qemu-ev: [Errno 256] No more mirrors to try.
http://mirror.centos.org/$contentdir/7/virt/x86_64/kvm-common/repodata/repomd.xml: [Errno 14] HTTP Error 404 - Not Foundgoogle找到如下解决办法，说是bug：
https://www.spinics.net/lists/centos-virt/msg05150.html
其中CentOS-QEMU-EV.repo文件存在问题，替换为旧文件问题解决，旧文件分享：
链接: https://pan.baidu.com/s/1He02ZQQiCn4kpAiyoQCQ7w 密码: k3hh
[root@localhost ~]# ll /etc/yum.repos.d/
total 48
-rw-r--r--. 1 root root 1664 Aug 30 2017 CentOS-Base.repo
-rw-r--r-- 1 root root 1095 Sep 21 2017 CentOS-Ceph-Luminous.repo
-rw-r--r--. 1 root root 1309 Aug 30 2017 CentOS-CR.repo
-rw-r--r--. 1 root root 649 Aug 30 2017 CentOS-Debuginfo.repo
-rw-r--r--. 1 root root 314 Aug 30 2017 CentOS-fasttrack.repo
-rw-r--r--. 1 root root 630 Aug 30 2017 CentOS-Media.repo
-rw-r--r-- 1 root root 1186 Feb 27 18:08 CentOS-OpenStack-queens.repo
-rw-r--r-- 1 root root 509 Jun 4 17:17 CentOS-QEMU-EV.repo
-rw-r--r--. 1 root root 1331 Aug 30 2017 CentOS-Sources.repo
-rw-r--r--. 1 root root 3830 Aug 30 2017 CentOS-Vault.repo
-rw-r--r--. 1 root root 2142 Jul 24 2017 elrepo.repo可以使用命令对比查看不一样的内容,问题可能出在contentdir上
[root@localhost ~]# diff -u CentOS-QEMU-EV.repo.old CentOS-QEMU-EV.repo
--- CentOS-QEMU-EV.repo.old 2018-06-04 17:17:04.535241765 +0800
+++ CentOS-QEMU-EV.repo 2018-05-10 22:33:42.000000000 +0800
@@ -5,7 +5,7 @@
[centos-qemu-ev]
name=CentOS-$releasever - QEMU EV
-baseurl=http://mirror.centos.org/centos/$releasever/virt/$basearch/kvm-common/
+baseurl=http://mirror.centos.org/$contentdir/$releasever/virt/$basearch/kvm-common/
gpgcheck=1
enabled=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-SIG-Virtualization也可以更换为fedora rdo源，但是使用这个源搭建到keystone时报错，不知道什么原因：
# wget https://repos.fedorapeople.org/repos/openstack/openstack-queens/rdo-release-queens-1.noarch.rpm
# rpm –ivh rdo-release-queens-1.noarch.rpm报错信息如下：
[root@controller ~]# openstack token issue
Failed to discover available identity versions when contacting http://controller:5000/v3. Attempting to parse version from URL.
Internal Server Error (HTTP 500)查看keystone日志
