何十何百台もHVを立てる時に、いちいち外部へyumしてるととても時間がかかるので
ローカルにリポジトリを作成しておくと良い。

# mkdir -p /var/www/html/mirror/centos
# rsync  -avSHP --delete --exclude "local*" --exclude "isos" rsync://ftp.iij.ad.jp/pub/linux/centos/6.7/ /var/www/html/mirror/centos/6.7/
# rsync http://ftp.iij.ad.jp/pub/linux/centos/RPM-GPG-KEY-CentOS-6 /var/www/html/mirror/centos/

[base]
name=CentOS-$releasever - Base
baseurl=http://[myrepo_name]/centos/$releasever/os/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-6

#released updates 
[updates]
name=CentOS-$releasever - Updates
baseurl=http://[myrepo_name]/centos/$releasever/updates/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-6
・
・
・

updateに追加の場合

# cd 
/var/www/html/mirror/centos/5.11/updates/x86_64
# ll
total 36
drwxr-xr-x 2 root root  4096 Apr 20 11:36 repodata
drwxrwxr-x 2 1000 1000 32768 Apr 20 11:27 RPMS

# cp kernel-2.6.18-409.el5.x86_64.rpm RPMS/

# createrepo --update ./
370/370 - RPMS/kmod-kvm-debug-83-272.el5.centos.x86_64.rpm                      
Saving Primary metadata
Saving file lists metadata
Saving other metadata

# yum clean all