diff --git a/content/en/blog/_posts/2023-05-04-statefulset-autodelete.md b/content/en/blog/_posts/2023-05-04-statefulset-autodelete.md new file mode 100644 index 00000000000..ae5e280c0bd --- /dev/null +++ b/content/en/blog/_posts/2023-05-04-statefulset-autodelete.md @@ -0,0 +1,109 @@ +--- +layout: blog +title: 'Kubernetes 1.27: StatefulSet PVC Auto-Deletion (beta)' +date: 2023-05-04 +slug: kubernetes-1-27-statefulset-pvc-auto-deletion-beta +--- + +**Author:** Matthew Cary (Google) + +Kubernetes v1.27 graduated to beta a new policy mechanism for +[`StatefulSets`](/docs/concepts/workloads/controllers/statefulset/) that controls the lifetime of +their [`PersistentVolumeClaims`](/docs/concepts/storage/persistent-volumes/) (PVCs). The new PVC +retention policy lets users specify if the PVCs generated from the `StatefulSet` spec template should +be automatically deleted or retrained when the `StatefulSet` is deleted or replicas 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 the PVC retention policy 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 + +The new `StatefulSet` PVC retention policy is used to control if and when PVCs created from a +`StatefulSet`’s `volumeClaimTemplate` are deleted. There are two contexts when this may occur. + +The first context is when the `StatefulSet` resource is deleted (which implies that all replicas are +also deleted). This is controlled by the `whenDeleted` policy. The second context, 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? + +Try it out! The `StatefulSetAutoDeletePVC` feature gate is beta and enabled by default on +cluster running Kubernetes 1.27. 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, I +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!