statefulset autodelete blog post
Matthew Cary
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layout: blog
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title: 'Kubernetes 1.23: StatefulSet PVC Auto-Deletion (alpha)'
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date: 2021-12-16
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slug: kubernetes-1-23-statefulset-pvc-auto-deletion
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---
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**Author:** Matthew Cary (Google)
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Kubernetes v1.23 introduced a new, alpha-level policy for
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[StatefulSets](docs/concepts/workloads/controllers/statefulset/) that controls the lifetime of
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[PersistentVolumeClaims](docs/concepts/storage/persistent-volumes/) (PVCs) generated from the
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StatefulSet spec template for cases when they should be deleted automatically when the StatefulSet
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is deleted or pods in the StatefulSet are scaled down.
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## What problem does this solve?
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A StatefulSet spec can include Pod and PVC templates. When a replica is first created, the
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Kubernetes control plane creates a PVC for that replica if one does not already exist. The behavior
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before Kubernetes v1.23 was that the control plane never cleaned up the PVCs created for
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StatefulSets - this was left up to the cluster administrator, or to some add-on automation that
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you’d have to find, check suitability, and deploy. The common pattern for managing PVCs, either
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manually or through tools such as Helm, is that the PVCs are tracked by the tool that manages them,
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with explicit lifecycle. Workflows that use StatefulSets must determine on their own what PVCs are
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created by a StatefulSet and what their lifecycle should be.
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Before this new feature, when a StatefulSet-managed replica disappears, either because the
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StatefulSet is reducing its replica count, or because its StatefulSet is deleted, the PVC and its
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backing volume remains and must be manually deleted. While this behavior is appropriate when the
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data is critical, in many cases the persistent data in these PVCs is either temporary, or can be
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reconstructed from another source. In those cases, PVCs and their backing volumes remaining after
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their StatefulSet or replicas have been deleted are not necessary, incur cost, and require manual
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cleanup.
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## The new StatefulSet PVC retention policy
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If you enable the alpha feature, a StatefulSet spec includes a PersistentVolumeClaim retention
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policy. This is used to control if and when PVCs created from a StatefulSet’s `volumeClaimTemplate`
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are deleted. This first iteration of the retention policy contains two situations where PVCs may be
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deleted.
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The first situation is when the StatefulSet resource is deleted (which implies that all replicas are
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also deleted). This is controlled by the `whenDeleted` policy. The second situation, controlled by
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`whenScaled` is when the StatefulSet is scaled down, which removes some but not all of the replicas
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in a StatefulSet. In both cases the policy can either be `Retain`, where the corresponding PVCs are
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not touched, or `Delete`, which means that PVCs are deleted. The deletion is done with a normal
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[object deletion](/docs/concepts/architecture/garbage-collection/), so that, for example, all
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retention policies for the underlying PV are respected.
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This policy forms a matrix with four cases. I’ll walk through and give an example for each one.
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* **`whenDeleted` and `whenScaled` are both `Retain`.** This matches the existing behavior for
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StatefulSets, where no PVCs are deleted. This is also the default retention policy. It’s
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appropriate to use when data on StatefulSet volumes may be irreplaceable and should only be
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deleted manually.
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* **`whenDeleted` is `Delete` and `whenScaled` is `Retain`.** In this case, PVCs are deleted only when
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the entire StatefulSet is deleted. If the StatefulSet is scaled down, PVCs are not touched,
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meaning they are available to be reattached if a scale-up occurs with any data from the previous
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replica. This might be used for a temporary StatefulSet, such as in a CI instance or ETL
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pipeline, where the data on the StatefulSet is needed only during the lifetime of the
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StatefulSet lifetime, but while the task is running the data is not easily reconstructible. Any
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retained state is needed for any replicas that scale down and then up.
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* **`whenDeleted` and `whenScaled` are both `Delete`.** PVCs are deleted immediately when their
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replica is no longer needed. Note this does not include when a Pod is deleted and a new version
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rescheduled, for example when a node is drained and Pods need to migrate elsewhere. The PVC is
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deleted only when the replica is no longer needed as signified by a scale-down or StatefulSet
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deletion. This use case is for when data does not need to live beyond the life of its
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replica. Perhaps the data is easily reconstructable and the cost savings of deleting unused PVCs
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is more important than quick scale-up, or perhaps that when a new replica is created, any data
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from a previous replica is not usable and must be reconstructed anyway.
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* **`whenDeleted` is `Retain` and `whenScaled` is `Delete`.** This is similar to the previous case,
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when there is little benefit to keeping PVCs for fast reuse during scale-up. An example of a
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situation where you might use this is an Elasticsearch cluster. Typically you would scale that
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workload up and down to match demand, whilst ensuring a minimum number of replicas (for example:
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3). When scaling down, data is migrated away from removed replicas and there is no benefit to
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retaining those PVCs. However, it can be useful to bring the entire Elasticsearch cluster down
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temporarily for maintenance. If you need to take the Elasticsearch system offline, you can do
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this by temporarily deleting the StatefulSet, and then bringing the Elasticsearch cluster back
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by recreating the StatefulSet. The PVCs holding the Elasticsearch data will still exist and the
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new replicas will automatically use them.
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Visit the
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[documentation](docs/concepts/workloads/controllers/statefulset/#persistentvolumeclaim-policies) to
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see all the details.
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## What’s next?
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Enable the feature and try it out! Enable the `StatefulSetAutoDeletePVC` feature gate on a cluster,
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then create a StatefulSet using the new policy. Test it out and tell us what you think!
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I'm very curious to see if this owner reference mechanism works well in practice. For example, we
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realized there is no mechanism in Kubernetes for knowing who set a reference, so it’s possible that
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the StatefulSet controller may fight with custom controllers that set their own
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references. Fortunately, maintaining the existing retention behavior does not involve any new owner
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references, so default behavior will be compatible.
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Please tag any issues you report with the label `sig/apps` and assign them to Matthew Cary
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([@mattcary](https://github.com/mattcary) at GitHub).
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Enjoy!
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