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Garbage collection is a collective term for the various mechanisms Kubernetes uses to clean up cluster resources. This allows the clean up of resources like the following:
- Terminated pods
- Completed Jobs
- Objects without owner references
- Unused containers and container images
- Dynamically provisioned PersistentVolumes with a StorageClass reclaim policy of Delete
- Stale or expired CertificateSigningRequests (CSRs)
- Nodes deleted in the following scenarios:
- On a cloud when the cluster uses a cloud controller manager
- On-premises when the cluster uses an addon similar to a cloud controller manager
- Node Lease objects
Owners and dependents
Many objects in Kubernetes link to each other through owner references. Owner references tell the control plane which objects are dependent on others. Kubernetes uses owner references to give the control plane, and other API clients, the opportunity to clean up related resources before deleting an object. In most cases, Kubernetes manages owner references automatically.
Ownership is different from the labels and selectors
mechanism that some resources also use. For example, consider a
Service that creates
EndpointSlice objects. The Service uses labels to allow the control plane to
EndpointSlice objects are used for that Service. In addition
to the labels, each
EndpointSlice that is managed on behalf of a Service has
an owner reference. Owner references help different parts of Kubernetes avoid
interfering with objects they don’t control.
Cross-namespace owner references are disallowed by design. Namespaced dependents can specify cluster-scoped or namespaced owners. A namespaced owner must exist in the same namespace as the dependent. If it does not, the owner reference is treated as absent, and the dependent is subject to deletion once all owners are verified absent.
Cluster-scoped dependents can only specify cluster-scoped owners. In v1.20+, if a cluster-scoped dependent specifies a namespaced kind as an owner, it is treated as having an unresolvable owner reference, and is not able to be garbage collected.
In v1.20+, if the garbage collector detects an invalid cross-namespace
or a cluster-scoped dependent with an
ownerReference referencing a namespaced kind, a warning Event
with a reason of
OwnerRefInvalidNamespace and an
involvedObject of the invalid dependent is reported.
You can check for that kind of Event by running
kubectl get events -A --field-selector=reason=OwnerRefInvalidNamespace.
Kubernetes checks for and deletes objects that no longer have owner references, like the pods left behind when you delete a ReplicaSet. When you delete an object, you can control whether Kubernetes deletes the object's dependents automatically, in a process called cascading deletion. There are two types of cascading deletion, as follows:
- Foreground cascading deletion
- Background cascading deletion
You can also control how and when garbage collection deletes resources that have owner references using Kubernetes finalizers.
Foreground cascading deletion
In foreground cascading deletion, the owner object you're deleting first enters a deletion in progress state. In this state, the following happens to the owner object:
- The Kubernetes API server sets the object's
metadata.deletionTimestampfield to the time the object was marked for deletion.
- The Kubernetes API server also sets the
- The object remains visible through the Kubernetes API until the deletion process is complete.
After the owner object enters the deletion in progress state, the controller deletes the dependents. After deleting all the dependent objects, the controller deletes the owner object. At this point, the object is no longer visible in the Kubernetes API.
During foreground cascading deletion, the only dependents that block owner
deletion are those that have the
See Use foreground cascading deletion
to learn more.
Background cascading deletion
In background cascading deletion, the Kubernetes API server deletes the owner object immediately and the controller cleans up the dependent objects in the background. By default, Kubernetes uses background cascading deletion unless you manually use foreground deletion or choose to orphan the dependent objects.
See Use background cascading deletion to learn more.
When Kubernetes deletes an owner object, the dependents left behind are called orphan objects. By default, Kubernetes deletes dependent objects. To learn how to override this behaviour, see Delete owner objects and orphan dependents.
Garbage collection of unused containers and images
The kubelet performs garbage collection on unused images every two minutes and on unused containers every minute. You should avoid using external garbage collection tools, as these can break the kubelet behavior and remove containers that should exist.
To configure options for unused container and image garbage collection, tune the
kubelet using a configuration file
and change the parameters related to garbage collection using the
Container image lifecycle
Kubernetes manages the lifecycle of all images through its image manager, which is part of the kubelet, with the cooperation of cadvisor. The kubelet considers the following disk usage limits when making garbage collection decisions:
Disk usage above the configured
HighThresholdPercent value triggers garbage
collection, which deletes images in order based on the last time they were used,
starting with the oldest first. The kubelet deletes images
until disk usage reaches the
Container garbage collection
The kubelet garbage collects unused containers based on the following variables, which you can define:
MinAge: the minimum age at which the kubelet can garbage collect a container. Disable by setting to
MaxPerPodContainer: the maximum number of dead containers each Pod can have. Disable by setting to less than
MaxContainers: the maximum number of dead containers the cluster can have. Disable by setting to less than
In addition to these variables, the kubelet garbage collects unidentified and deleted containers, typically starting with the oldest first.
MaxContainers may potentially conflict with each other
in situations where retaining the maximum number of containers per Pod
MaxPerPodContainer) would go outside the allowable total of global dead
MaxContainers). In this situation, the kubelet adjusts
MaxPerPodContainer to address the conflict. A worst-case scenario would be to
1 and evict the oldest containers.
Additionally, containers owned by pods that have been deleted are removed once
they are older than
Configuring garbage collection
You can tune garbage collection of resources by configuring options specific to the controllers managing those resources. The following pages show you how to configure garbage collection: