Kubelet的GC功能将清理未使用的image和container。Kubelet每分钟对container执行一次GC，每5分钟对image执行一次GC。不建议使用外部垃圾收集工具，因为这些工具可能破坏Kubelet。
image
Kubernetes与Cadvisor配合，通过ImageManager管理所有image的生命周期。
image的GC策略包含高阈值和低阈值，高于高阈值的磁盘使用率将触发GC,删除最近最少使用的图像，直到满足低阈值为止。
container
container的GC主要有3个用户定义变量：
MinAge：容器被GC的最短时间
MaxPerPodContainer: 允许每个PodContainer中死容器的最大数目,PodContainer指1个Container而非pod
MaxContainers：死容器的最大数目
Minage=0，MaxPerPodContainer和MaxContainers <0, 表示禁用这些变量
GC用于unidentified、deleted或超出边界的容器(3个用户定义变量)。
最旧的container通常首先被移除。
如果MaxPerPodContainer>MaxContainers,maxperpodcontainer会进行调整，直至降级为1，并逐出最旧的容器。
pods所拥有的已删除的容器一旦超过MinAge，就会被删除。
未由Kubelet管理的容器不受容器垃圾收集的约束。
配置 GC
通过修改kubelet flags来实现。
image gc参数：
image-gc-high-threshold image GC 高阈值百分比，缺省为85%
image-gc-low-threshold image GC 低阈值百分比，缺省为80%
minimum-container-ttl-duration MinAge参数，缺省为0
maximum-dead-containers-per-container MaxPerPodContainer，缺省为1.
maximum-dead-containers MaxContainers，缺省为-1, 也就是没有限制
Container可能在其到期之前被GC,Container包含日志和其他对故障排除有用的数据。强烈建议MaxPerPodContainer和maximum-dead-containers 足够大。
GC效果
配置一个10s后会失败的container 配置文件pod-gc.yaml
apiVersion: v1
kind: Pod
metadata:
name: gc-test
spec:
containers:
- name: busybox-gc-1
image: busybox:v1
command:
- /bin/sh
- -c
- 'sleep 10 && hello'
使用yaml创建pod
kubectl apply -f pod-gc.yaml
查看docker的状态，当出现2个exit的container时，就会出发GC,回收最早的一个container
# docker ps -a | grep busybox-gc
8ddbfcf4ebee 59788edf1f3e "/bin/sh -c 'sleep 1…" 2 seconds ago Up 1 second k8s_busybox-gc-1_gc-test_default_3651c167-75f0-11e9-bc74-52540005f38a_5
7d7c1ade82af 59788edf1f3e "/bin/sh -c 'sleep 1…" About a minute ago Exited (127) About a minute ago k8s_busybox-gc-1_gc-test_default_3651c167-75f0-11e9-bc74-52540005f38a_4
# docker ps -a | grep busybox-gc
8ddbfcf4ebee 59788edf1f3e "/bin/sh -c 'sleep 1…" 10 seconds ago Up 9 seconds k8s_busybox-gc-1_gc-test_default_3651c167-75f0-11e9-bc74-52540005f38a_5
7d7c1ade82af 59788edf1f3e "/bin/sh -c 'sleep 1…" About a minute ago Exited (127) About a minute ago k8s_busybox-gc-1_gc-test_default_3651c167-75f0-11e9-bc74-52540005f38a_4
# docker ps -a | grep busybox-gc
8ddbfcf4ebee 59788edf1f3e "/bin/sh -c 'sleep 1…" 11 seconds ago Exited (127) Less than a second ago k8s_busybox-gc-1_gc-test_default_3651c167-75f0-11e9-bc74-52540005f38a_5
7d7c1ade82af 59788edf1f3e "/bin/sh -c 'sleep 1…" About a minute ago Exited (127) About a minute ago k8s_busybox-gc-1_gc-test_default_3651c167-75f0-11e9-bc74-52540005f38a_4
# docker ps -a | grep busybox-gc
8ddbfcf4ebee 59788edf1f3e "/bin/sh -c 'sleep 1…" 12 seconds ago Exited (127) 1 second ago k8s_busybox-gc-1_gc-test_default_3651c167-75f0-11e9-bc74-52540005f38a_5
修改配置文件/var/lib/kubelet/kubeadm-flags.env增加参数--maximum-dead-containers-per-container=2
KUBELET_KUBEADM_ARGS=--cgroup-driver=systemd --network-plugin=cni --pod-infra-container-image=k8s.gcr.io/pause:3.1 --maximum-dead-containers-per-container=2
重启kubelet服务
systemctl restart kubelet
删除前面的pod
kubectl delete -f pod-gc.yaml
重新添加pod
kubectl apply -f pod-gc.yaml
可以看到，当exit的容器达到3个时，才会触发gc
# docker ps -a | grep busybox-gc
c8677098e9aa af2f74c517aa "/bin/sh -c 'sleep 1…" 8 seconds ago Up 8 seconds k8s_busybox-gc-1_gc-test_default_3425a55b-75f5-11e9-bc74-52540005f38a_2
f0bc1b7893bd af2f74c517aa "/bin/sh -c 'sleep 1…" 34 seconds ago Exited (127) 23 seconds ago k8s_busybox-gc-1_gc-test_default_3425a55b-75f5-11e9-bc74-52540005f38a_1
9e7fa118b1d3 af2f74c517aa "/bin/sh -c 'sleep 1…" 45 seconds ago Exited (127) 35 seconds ago k8s_busybox-gc-1_gc-test_default_3425a55b-75f5-11e9-bc74-52540005f38a_0
# docker ps -a | grep busybox-gc
c8677098e9aa af2f74c517aa "/bin/sh -c 'sleep 1…" 9 seconds ago Up 9 seconds k8s_busybox-gc-1_gc-test_default_3425a55b-75f5-11e9-bc74-52540005f38a_2
f0bc1b7893bd af2f74c517aa "/bin/sh -c 'sleep 1…" 35 seconds ago Exited (127) 24 seconds ago k8s_busybox-gc-1_gc-test_default_3425a55b-75f5-11e9-bc74-52540005f38a_1
9e7fa118b1d3 af2f74c517aa "/bin/sh -c 'sleep 1…" 46 seconds ago Exited (127) 36 seconds ago k8s_busybox-gc-1_gc-test_default_3425a55b-75f5-11e9-bc74-52540005f38a_0
# docker ps -a | grep busybox-gc
c8677098e9aa af2f74c517aa "/bin/sh -c 'sleep 1…" 10 seconds ago Exited (127) Less than a second ago k8s_busybox-gc-1_gc-test_default_3425a55b-75f5-11e9-bc74-52540005f38a_2
f0bc1b7893bd af2f74c517aa "/bin/sh -c 'sleep 1…" 36 seconds ago Exited (127) 25 seconds ago k8s_busybox-gc-1_gc-test_default_3425a55b-75f5-11e9-bc74-52540005f38a_1
9e7fa118b1d3 af2f74c517aa "/bin/sh -c 'sleep 1…" 47 seconds ago Exited (127) 36 seconds ago k8s_busybox-gc-1_gc-test_default_3425a55b-75f5-11e9-bc74-52540005f38a_0
# docker ps -a | grep busybox-gc
c8677098e9aa af2f74c517aa "/bin/sh -c 'sleep 1…" 11 seconds ago Exited (127) 1 second ago k8s_busybox-gc-1_gc-test_default_3425a55b-75f5-11e9-bc74-52540005f38a_2
f0bc1b7893bd af2f74c517aa "/bin/sh -c 'sleep 1…" 37 seconds ago Exited (127) 26 seconds ago k8s_busybox-gc-1_gc-test_default_3425a55b-75f5-11e9-bc74-52540005f38a_1
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