Merge pull request #32659 from pohly/kubernetes-1.24-storage-capacity-ga

storage capacity GA blog post

Author: k8s-ci-robot

content/en/blog/_posts/2022-05-06-storage-capacity-GA/index.md

@@ -0,0 +1,79 @@
+---
+layout: blog
+title: "Storage Capacity Tracking reaches GA in Kubernetes 1.24"
+date: 2022-05-06
+slug: storage-capacity-ga
+---
+
+ **Authors:** Patrick Ohly (Intel)
+
+The v1.24 release of Kubernetes brings [storage capacity](/docs/concepts/storage/storage-capacity/)
+tracking as a generally available feature.
+
+## Problems we have solved
+
+As explained in more detail in the [previous blog post about this
+feature](/blog/2021/04/14/local-storage-features-go-beta/), storage capacity
+tracking allows a CSI driver to publish information about remaining
+capacity. The kube-scheduler then uses that information to pick suitable nodes
+for a Pod when that Pod has volumes that still need to be provisioned.
+
+Without this information, a Pod may get stuck without ever being scheduled onto
+a suitable node because kube-scheduler has to choose blindly and always ends up
+picking a node for which the volume cannot be provisioned because the
+underlying storage system managed by the CSI driver does not have sufficient
+capacity left.
+
+Because CSI drivers publish storage capacity information that gets used at a
+later time when it might not be up-to-date anymore, it can still happen that a
+node is picked that doesn't work out after all. Volume provisioning recovers
+from that by informing the scheduler that it needs to try again with a
+different node.
+
+[Load
+tests](https://github.com/kubernetes-csi/csi-driver-host-path/blob/master/docs/storage-capacity-tracking.md)
+that were done again for promotion to GA confirmed that all storage in a
+cluster can be consumed by Pods with storage capacity tracking whereas Pods got
+stuck without it.
+
+## Problems we have *not* solved
+
+Recovery from a failed volume provisioning attempt has one known limitation: if a Pod
+uses two volumes and only one of them could be provisioned, then all future
+scheduling decisions are limited by the already provisioned volume. If that
+volume is local to a node and the other volume cannot be provisioned there, the
+Pod is stuck. This problem pre-dates storage capacity tracking and while the
+additional information makes it less likely to occur, it cannot be avoided in
+all cases, except of course by only using one volume per Pod.
+
+An idea for solving this was proposed in a [KEP
+draft](https://github.com/kubernetes/enhancements/pull/1703): volumes that were
+provisioned and haven't been used yet cannot have any valuable data and
+therefore could be freed and provisioned again elsewhere. SIG Storage is
+looking for interested developers who want to continue working on this.
+
+Also not solved is support in Cluster Autoscaler for Pods with volumes. For CSI
+drivers with storage capacity tracking, a prototype was developed and discussed
+in [a PR](https://github.com/kubernetes/autoscaler/pull/3887). It was meant to
+work with arbitrary CSI drivers, but that flexibility made it hard to configure
+and slowed down scale up operations: because autoscaler was unable to simulate
+volume provisioning, it only scaled the cluster by one node at a time, which
+was seen as insufficient.
+
+Therefore that PR was not merged and a different approach with tighter coupling
+between autoscaler and CSI driver will be needed. For this a better
+understanding is needed about which local storage CSI drivers are used in
+combination with cluster autoscaling. Should this lead to a new KEP, then users
+will have to try out an implementation in practice before it can move to beta
+or GA. So please reach out to SIG Storage if you have an interest in this
+topic.
+
+## Acknowledgements
+
+Thanks a lot to the members of the community who have contributed to this
+feature or given feedback including members of [SIG
+Scheduling](https://github.com/kubernetes/community/tree/master/sig-scheduling),
+[SIG
+Autoscaling](https://github.com/kubernetes/community/tree/master/sig-autoscaling),
+and of course [SIG
+Storage](https://github.com/kubernetes/community/tree/master/sig-storage)!