考虑k8s下docker下载镜像太慢，修改了daemon.json，按照手册抄，添加
{
"exec-opts": ["native.cgroupdriver=systemd"],
"registry-mirrors": ["https://kn0t2bca.mirror.aliyuncs.com"]
}
结果发现k8s起不来了，
-- Unit docker.service has begun starting up.
Aug 10 22:02:01 k8s-master dockerd[1831]: unable to configure the Docker daemon with file /etc/docker/daemon.json : the following directives are specified both as a flag and in the configuration file: exec-opts : (from flag: [native.cgroupdriver=systemd] , from file: [native.cgroupdriver=systemd] )
Aug 10 22:02:01 k8s-master systemd[1]: docker.service: main process exited, code=exited, status=1/FAILURE
Aug 10 22:02:01 k8s-master systemd[1]: Failed to start Docker Application Container Engine.
-- Subject: Unit docker.service has failed
-- Defined-By: systemd
-- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
--
-- Unit docker.service has failed.
--
-- The result is failed.
Aug 10 22:02:01 k8s-master systemd[1]: Unit docker.service entered failed state.
Aug 10 22:02:01 k8s-master systemd[1]: docker.service failed.
原因
启动失败原因：直接启动docker会报错，因为docker.service里有一条配置，和刚才添加的"exec-opts"冲突了
# /lib/systemd/system/docker.service
ExecStart=/usr/bin/dockerd -H fd:// --containerd=/run/containerd/containerd.sock --exec-opt native.cgroupdriver=systemd
解决方案，保留一处正确的即可：
1.1. # docker.service 找到并删除下面这句话，保存退出，即可解决
--exec-opt native.cgroupdriver=cgroupfs \
1.2. # 或者daemon.json
删exec-opt所在行
init 6 重启
重启OK