1.磁盘使用率达到85时，触发imageGCManager
现象：
node event：
(combined from similar events): failed to garbage collect required amount of images. Wanted to free 50015916032 bytes, but freed 0 bytes
Source
message:
kubelet: I0629 10:56:28.575434 107257 image_gc_manager.go:371] [imageGCManager]: Removing image "sha256:471f9537bf3685f3befcd3a6d86f3ad991d5ae6e03a112de1d66278dc979037c" to free 1108218030 bytes
kubelet: E0629 10:56:29.117032 107257 kubelet.go:1298] Image garbage collection failed multiple times in a row: failed to garbage collect required amount of images. Wanted to free 45641629696 bytes, but freed 1129986950 bytes
解释：
当kubelet检测到磁盘使用率达到阀值，就会自动清理过期的image，释放空间，这个阀值可通过kubelet参数调整。
$ kubelet --help | grep 85
--image-gc-high-threshold int32 The percent of disk usage after which image garbage collection is always run. Values must be within the range [0, 100], To disable image garbage collection, set to 100. (default 85) (DEPRECATED: This parameter should be set via the config file specified by theKubelet‘s --config flag. See https://kubernetes.io/docs/tasks/administer-cluster/kubelet-config-file/ for more information.)