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Many of my workflows use pod iam roles. As documented here, I must include fsGroup in order for non-root containers to read the generated identity token. The problem with this is when I additionally include pvc’s that point to cifs pv’s, the volumes fail to mount because they time out. Seemingly this is because Kubelet tries to chown all of the files on the volume, which takes too much time and causes the timeout. Questions…

  1. Why doesnt Kubernetes try to chown all of the files when hostPath is used instead of a pvc? All of the workflows were fine until I made the switch to use pvcs from hostPath, and now the timeout issue happens.
  2. Why does this problem happen on cifs pvcs but not nfs pvcs? I have noticed that nfs pvcs continue to mount just fine and the fsGroup seemingly doesn’t take effect as I don’t see the group id change on any of the files. However, the cifs pvcs can no longer be mounted seemingly due to the timeout issue. If it matters, I am using the native nfs pv lego and this cifs flexVolume plugin that has worked great up until now.

Overall, the goal of this post is to better understand how Kubernetes determines when to chown all of the files on a volume when fsGroup is included in order to make a good design decision going forward. Thanks for any help you can provide!

Kubernetes Chowning Files References

https://learn.microsoft.com/en-us/azure/aks/troubleshooting

Since gid and uid are mounted as root or 0 by default. If gid or uid are set as non-root, for example 1000, Kubernetes will use chown to change all directories and files under that disk. This operation can be time consuming and may make mounting the disk very slow.

https://kubernetes.io/docs/tasks/configure-pod-container/security-context/#configure-volume-permission-and-ownership-change-policy-for-pods

By default, Kubernetes recursively changes ownership and permissions for the contents of each volume to match the fsGroup specified in a Pod's securityContext when that volume is mounted. For large volumes, checking and changing ownership and permissions can take a lot of time, slowing Pod startup.

Howard_Roark
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  • Hello Kubernetes Community- If anyone has any thoughts on this, please let me know. I have tried asking in the Kubernetes Discussion Forum and I was about to ask on the Kubernetes github repo itself-- but it points out that Support Related questions should be asked here. I have also tried speaking with AWS representatives who know Kubernetes fairly well about this, and in general so far no one has been able to explain how this works – Howard_Roark Aug 07 '20 at 18:00

1 Answers1

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I posted this question on the Kubernetes Repo a while ago and it was recently answered in the comments.

The gist is fsgroup support is implemented and decided on per plugin. They ignore it for nfs, which is why I have never seen Kubelet chown files on nfs pvcs. For FlexVolume plugins, a plugin can opt-out of fsGroup based permission changes by returning FSGroup false. So, that is why Kubelet was trying to chown the cifs pvcs -- the FlexVolume plugin I am using does not return fsGroup false.

So, in the end you don't need to worry about this for nfs, and if you are using a FlexVolume plugin for a shared file system, you should make sure it returns fsGroup false if you don't want Kubelet to chown all of the files.

Howard_Roark
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