Merge pull request #30596 from mattcary/blogpost

StatefulSet AutoDelete blog post

Author: k8s-ci-robot

content/en/blog/_posts/2021-12-16-StatefulSet-PVC-Auto-Deletion.md

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