K8s新手系列之Pod中容器的镜像拉取策略
概述在 Kubernetes（K8s）里，容器镜像拉取策略（ImagePullPolicy）决定了 K8s 在创建或重启 Pod 时，如何处理容器镜像的拉取操作。这一策略能够确保使用的镜像始终是最新的，或者使用本地已有的镜像以提升部署效率。
可以使用kubectl explain pod.spec.containers.imagePullPolicy查看资源的详细文档：
示例：
# kubectl explain pod.spec.containers.imagePullPolicy
KIND: Pod
VERSION:v1
FIELD: imagePullPolicy <string>
DESCRIPTION:
Image pull policy. One of Always, Never, IfNotPresent. Defaults to Always
if :latest tag is specified, or IfNotPresent otherwise. Cannot be updated.
More info:
https://kubernetes.io/docs/concepts/containers/images#updating-images
Possible enum values:
- `"Always"` means that kubelet always attempts to pull the latest image.
Container will fail If the pull fails.
- `"IfNotPresent"` means that kubelet pulls if the image isn't present on
disk. Container will fail if the image isn't present and the pull fails.
- `"Never"` means that kubelet never pulls an image, but only uses a local
image. Container will fail if the image isn't present通过文档可以发现，镜像拉取策略有三个类型，分别是Always、IfNotPresent、Never
也可以通过官方文档来查看：https://kubernetes.io/docs/concepts/containers/images#updating-images
镜像拉取策略分类详解
Never
Never表示永不拉取镜像，kubelet 不会尝试获取镜像。如果镜像已经以某种方式存在本地， kubelet 会尝试启动容器；否则，会启动失败。
示例：
# cat pod-tomcat.yaml
apiVersion: v1
kind: Pod
metadata:
name: pod-tomcat
spec:
containers:
- name: container-tomcat
image: tomcat:8.0
# 指定镜像拉取策略为Never
imagePullPolicy: Never
ports:
- name: http
containerPort: 8080
# 创建pod
# kubectl apply -f pod-tomcat.yaml
pod/pod-tomcat created
# 查看pod
# kubectl get po pod-tomcat
NAME READY STATUS RESTARTS AGE
pod-tomcat 0/1 ErrImageNeverPull 0 111s
# 查看详情，发现并没有拉取镜像
# kubectl describe po pod-tomcat
Name: pod-tomcat
...省略万字内容
Events:
Type Reason Age From Message
---- ------ ---- ---- -------
Normal Scheduled 2m7s default-schedulerSuccessfully assigned default/pod-tomcat to node02
Normal SandboxChanged 2m5s kubelet Pod sandbox changed, it will be killed and re-created.
WarningErrImageNeverPull25s (x12 over 2m6s)kubelet Container image "tomcat:8.0" is not present with pull policy of Never
WarningFailed 25s (x12 over 2m6s)kubelet Error: ErrImageNeverPullIfNotPresent
如果本地有镜像就使用本地的，如果没有就去远程仓库拉取
示例：
# cat pod-tomcat.yaml
apiVersion: v1
kind: Pod
metadata:
name: pod-tomcat
spec:
containers:
- name: container-tomcat
image: tomcat:8.0
# 指定镜像拉取策略为IfNotPresent
imagePullPolicy: IfNotPresent
ports:
- name: http
containerPort: 8080
# 创建pod
# kubectl apply -f pod-tomcat.yaml
pod/pod-tomcat created
# 查看pod
# kubectl get pod pod-tomcat
NAME READY STATUS RESTARTS AGE
pod-tomcat 1/1 Running 0 43s
#查看pod详情
# kubectl describe pod pod-tomcat
Name: pod-tomcat
Namespace: default
##...省略万字内容
Events:
Type Reason Age From Message
---- ------ -------- -------
NormalScheduled64s default-schedulerSuccessfully assigned default/pod-tomcat to node02
NormalPulling 63s kubelet Pulling image "tomcat:8.0"
NormalPulled 23s kubelet Successfully pulled image "tomcat:8.0" in 40.129798231s (40.129811976s including waiting)
NormalCreated 23s kubelet Created container container-tomcat
NormalStarted 23s kubelet Started container container-tomcatAlways
如果本地有镜像，则对比本地镜像和远程仓库的摘要信息,若相同则使用本地缓存，若不同则重新拉取镜像。
如果本地没有镜像，则无需对比摘要信息，直接拉取镜像。
示例：
# cat pod-tomcat.yaml
apiVersion: v1
kind: Pod
metadata:
name: pod-tomcat
spec:
containers:
- name: container-tomcat
# 镜像更改为latest
image: tomcat:latest
# 指定镜像拉取策略为Always
imagePullPolicy: Always
ports:
- name: http
containerPort: 8080
# 创建pod
# kubectl apply -f pod-tomcat.yaml
pod/pod-tomcat created
#查看pod
# kubectl get pod pod-tomcat
NAME READY STATUS RESTARTS AGE
pod-tomcat 1/1 Running 0 4m53s
# 查看pod详情
# kubectl describe pod pod-tomcat
Name: pod-tomcat
Namespace: default
## ...省略万字内容
Events:
Type Reason Age From Message
---- ------ ---- ---- -------
NormalScheduled5m39sdefault-schedulerSuccessfully assigned default/pod-tomcat to node02
NormalPulling 5m38skubelet Pulling image "tomcat:latest"
NormalPulled 4m55skubelet Successfully pulled image "tomcat:latest" in 43.43020764s (43.430224485s including waiting)
NormalCreated 4m55skubelet Created container container-tomcat
NormalStarted 4m55skubelet Started container container-tomcat镜像默认的拉取策略
官网的描述是这样的：
[*]如果你省略了 imagePullPolicy 字段，并且你为容器镜像指定了摘要， 那么 imagePullPolicy 会自动设置为 IfNotPresent。
[*]如果你省略了 imagePullPolicy 字段，并且容器镜像的标签是 :latest， imagePullPolicy 会自动设置为 Always。
[*]如果你省略了 imagePullPolicy 字段，并且没有指定容器镜像的标签， imagePullPolicy 会自动设置为 Always。
[*]如果你省略了 imagePullPolicy 字段，并且为容器镜像指定了非 :latest 的标签， imagePullPolicy 就会自动设置为 IfNotPresent
简单点来说就是如果没有指定镜像的版本，或者指定镜像的版本为 :latest，那么镜像拉取策略就是 Always。否则就是 IfNotPresent
来源：程序园用户自行投稿发布，如果侵权，请联系站长删除
免责声明：如果侵犯了您的权益，请联系站长，我们会及时删除侵权内容，谢谢合作！
页:
[1]