Use image caches to accelerate pod creation for Knative Services - Container Service for Kubernetes

When you deploy Knative services from container images, large images can slow pod creation during the pull phase. Use ECI ImageCache to pre-build image snapshots and create pods from them, reducing startup time.

For example, pulling a ~386.26 MB Apache Flink image from Docker Hub takes about 50 seconds without a cache, but about 5 seconds with one.

These figures are theoretical. Actual savings depend on image count, size, and network connection to the registry.

How it works

In an ACK cluster, image caches are managed as a cluster-level CRD named ImageCache. The ImageCache CRD is shared across all namespaces.

Create a cache snapshot from an image, then use it to deploy a pod on an elastic container instance. This reduces image layers to download and accelerates pod creation.

See Overview of image caches for management and billing.

Prerequisites

Ensure that you have:

An ACK cluster with Knative enabled
kubectl configured to connect to your cluster

Create an ImageCache

Create a file named imagecache-secrets-test.yaml:

The k8s.aliyun.com/eci-image-cache: "true" annotation enables reuse of existing ImageCaches. If the images to cache share layers with an existing ImageCache, this speeds up caching. See Manage ImageCaches for all configurable parameters.

apiVersion: eci.alibabacloud.com/v1
kind: ImageCache
metadata:
  name: imagecache-sample-test
  annotations:
    k8s.aliyun.com/eci-image-cache: "true" # Enable the reuse of ImageCaches.
spec:
  images:
  - registry.cn-shanghai.aliyuncs.com/eci_open/nginx:1.14.2
  - registry.cn-shanghai.aliyuncs.com/eci_open/busybox:1.30
  imageCacheSize:
   25
  retentionDays:
   7

Create the ImageCache:

kubectl create -f imagecache-secrets-test.yaml

Verify the ImageCache is ready:

kubectl get imagecache imagecache-sample-test

The ImageCache is ready when PHASE shows Ready and PROGRESS shows 100%:

NAME                              AGE   CACHEID                               PHASE   PROGRESS
imagecache-sample-test            20h   imc-2zeditzeoemfhqor****              Ready    100%

Use an ImageCache to accelerate pod creation

Annotate the Knative Service pod metadata to specify the ImageCache. Two annotations:

Annotation	Behavior
`k8s.aliyun.com/eci-image-snapshot-id`	Uses the specified ImageCache
`k8s.aliyun.com/eci-image-cache`	Automatically selects the best available ImageCache

If both are set, only k8s.aliyun.com/eci-image-snapshot-id takes effect.

Specify an ImageCache

Use the k8s.aliyun.com/eci-image-snapshot-id annotation to pin a Knative Service to a specific ImageCache.

Important

The ImageCache must be in Ready state before you create the pod. Pods referencing a non-ready ImageCache fail to start.

apiVersion: serving.knative.dev/v1
kind: Service
metadata:
  name: helloworld-go
spec:
  template:
    metadata:
      labels:
        app: helloworld-go
      annotations:
        k8s.aliyun.com/eci-image-snapshot-id: imc-2ze5tm5gehgtiiga****  # Specify the ImageCache that you want to use.
    spec:
      containers:
        - image: registry.cn-hangzhou.aliyuncs.com/knative-sample/helloworld-go:73fbdd56

Automatically select an ImageCache

Use the k8s.aliyun.com/eci-image-cache: "true" annotation to let ECI select the best matching ImageCache automatically.

ECI filters ImageCaches in the region that fit the instance's temporary storage, then selects one by priority:

Match degree — The cache that best matches the pod's image registry and version.
Cache size — Among equal matches, the cache closest in size to the image.
Creation time — If tied, the most recently created cache.

If no cache matches, ECI pulls and caches the image during pod creation. Set the image pull policy to IfNotPresent for caching efficiency.

apiVersion: serving.knative.dev/v1
kind: Service
metadata:
  name: helloworld-go
spec:
  template:
    metadata:
      labels:
        app: helloworld-go
      annotations:
        k8s.aliyun.com/eci-image-cache: "true"    # Automatically select an ImageCache.
    spec:
      containers:
        - image: registry.cn-hangzhou.aliyuncs.com/knative-sample/helloworld-go:73fbdd56

Next steps

Manage ImageCaches — Create, query, and delete ImageCaches
Image caches billing — Billing methods and pricing