k8s滚动更新时删除pod的规则

k8s 在弹性缩容或滚动更新时默认的pod删除规则如下：

1 如果pod没分配到节点，先被删除
2 如果pod的状态是 Pending>PodUnknown>PodRunning，则Pending优先被删除，PodUnknown次之，PodRunning最后被删除。
3 不是Ready状态的Pod先被删除。
4 如果Pod都是Ready状态，则最后一个变成Ready状态的Pod先被删除（Ready时间最短的）。
5 重启次数大的Pod先被删除。
6 创建时间最新的Pod先被删除。

参考代码

func (s ActivePods) Less(i, j int) bool {
    // 1. Unassigned < assigned
    // If only one of the pods is unassigned, the unassigned one is smaller
    if s[i].Spec.NodeName != s[j].Spec.NodeName && (len(s[i].Spec.NodeName) == 0 || len(s[j].Spec.NodeName) == 0) {
        return len(s[i].Spec.NodeName) == 0
    }
    // 2. PodPending < PodUnknown < PodRunning
    m := map[v1.PodPhase]int{v1.PodPending: 0, v1.PodUnknown: 1, v1.PodRunning: 2}
    if m[s[i].Status.Phase] != m[s[j].Status.Phase] {
        return m[s[i].Status.Phase] < m[s[j].Status.Phase]
    }
    // 3. Not ready < ready
    // If only one of the pods is not ready, the not ready one is smaller
    if podutil.IsPodReady(s[i]) != podutil.IsPodReady(s[j]) {
        return !podutil.IsPodReady(s[i])
    }
    // TODO: take availability into account when we push minReadySeconds information from deployment into pods,
    //       see https://github.com/kubernetes/kubernetes/issues/22065
    // 4. Been ready for empty time < less time < more time
    // If both pods are ready, the latest ready one is smaller
    if podutil.IsPodReady(s[i]) && podutil.IsPodReady(s[j]) && !podReadyTime(s[i]).Equal(podReadyTime(s[j])) {
        return afterOrZero(podReadyTime(s[i]), podReadyTime(s[j]))
    }
    // 5. Pods with containers with higher restart counts < lower restart counts
    if maxContainerRestarts(s[i]) != maxContainerRestarts(s[j]) {
        return maxContainerRestarts(s[i]) > maxContainerRestarts(s[j])
    }
    // 6. Empty creation time pods < newer pods < older pods
    if !s[i].CreationTimestamp.Equal(&s[j].CreationTimestamp) {
        return afterOrZero(&s[i].CreationTimestamp, &s[j].CreationTimestamp)
    }
    return false
}