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Based on this blog post https://blogs.technet.microsoft.com/uspartner_ts2team/2015/08/26/azure-vm-drive-attachment-limits/ there is a limit on the disk attachment following the model of number of cpus x2. Is there a technical reason why this limit is in place? If you use kubernetes you may not be able to schedule a pod. The scheduler is not aware of this limit.

This was proposed as a workaround https://github.com/khenidak/dysk but I'm wondering why this very low limit exists in the first place.

Argy
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The number of data disks are directly tied to the size of the VM. For example, if you go here https://learn.microsoft.com/en-us/azure/virtual-machines/windows/sizes you will see that each VM increasing in resources can handle more data disks.

This restraint is mainly built around performance. If you had a virtual machine with only 2 CPU cores and say 10 data disks you would likely run into performance issues as the CPU power and RAM needed to reach out to all those data disks at once could cause your VM to tap out.

The simple solution would be to use larger VM sizes if you need more disks. Or depending on how much space you have Azure can support up to 4TB data disks.

micahmckittrick
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  • thats totally understandable. The issue lies within a kuberentes environment. You cannot just use the larger instances because you will have huge costs without taking advantage of the full potential of the box. Probably we need a way to make kuberentes aware about those limitations in order to schedule pods somewhere else. Probably instead of a hard limit there could be an automated alert or something. We are currently doing this with a custom process but it will be good if Azure offers this out of the box. – Argy Oct 31 '18 at 17:10
  • That's good feedback. I completely agree. I would suggest leaving that feedback on our uservoice page https://feedback.azure.com/forums/914020-azure-kubernetes-service-aks this allows others to also upvote your idea and then the product team can consider adding that functionality. – micahmckittrick Oct 31 '18 at 17:21
  • Thank you and posted https://feedback.azure.com/forums/914020-azure-kubernetes-service-aks/suggestions/35881294-let-kubernetes-be-aware-of-data-disk-attachment-li – Argy Nov 01 '18 at 13:15