If
you select a VM SKU that supports Ephemeral OS disks but don't specify an OS
disk size, AKS by default provisions an Ephemeral OS disk with a size that
scales according to the total temp storage of the VM SKU so long as the temp is
at least 128GiB. If you want to use the temp storage of the VM SKU, you need to
specify the OS disk size during deployment, otherwise it’s consumed by default.
Learn more.