Feature: Sum all non-root EBS volumes for ephemeral-storage capacity calculation

Description

What problem are you trying to solve? We're running Bottlerocket nodes with multiple EBS volumes that are aggregated via RAID0 (using a bootstrap container). Karpenter's ephemeralStorage() function returns only a single block device size on nodeclaims, not reflecting the actual storage available to pods after our userdata aggregates the volumes.

Code reference: https://github.com/aws/karpenter-provider-aws/blob/10c4ef30c58b54476f67deff8f62fe81b31c415b/pkg/providers/instancetype/types.go#L358-L392

What the Bootstrap Container Does:

Detects all non-root EBS volumes: /dev/xvdb (18Gi), /dev/xvde (20Gi), /dev/xvdf (20Gi), /dev/xvdg (20Gi) Creates RAID0 array from all 4 volumes Mounts combined storage (78Gi total) to /data Bind mounts /var/lib/kubelet and /var/lib/containerd to RAID array

The Gap: Karpenter calculates for nodeclaim: 18Gi (only /dev/xvdb - the EphemeralBlockDevice) Actual available storage on node: 78Gi (RAID0 of all 4 non-root volumes) Bin-packing decisions are based on incorrect capacity, causing premature scaling.

Example EC2NodeClass configuration (Bottlerocket):

apiVersion: karpenter.k8s.aws/v1
kind: EC2NodeClass
spec:
  amiFamily: Bottlerocket
  blockDeviceMappings:
    - deviceName: /dev/xvda          # Root volume: 2Gi (OS only)
      ebs:
        volumeSize: 2Gi
        volumeType: gp3
        encrypted: true
    - deviceName: /dev/xvdb          # EphemeralBlockDevice: 18Gi
      ebs:
        volumeSize: 18Gi
        volumeType: gp3
        encrypted: true
    - deviceName: /dev/xvde          # Additional: 20Gi
      ebs:
        volumeSize: 20Gi
        volumeType: gp3
        encrypted: true
    - deviceName: /dev/xvdf          # Additional: 20Gi
      ebs:
        volumeSize: 20Gi
        volumeType: gp3
        encrypted: true
    - deviceName: /dev/xvdg          # Additional: 20Gi
      ebs:
        volumeSize: 20Gi
        volumeType: gp3
        encrypted: true

Question:

How should we handle scenarios where userdata/bootstrap scripts aggregate multiple volumes?

Annotation-based override - e.g., karpenter.sh/ephemeral-storage-override: "78Gi" New field in EC2NodeClass - e.g., spec.ephemeralStorageCapacity: 78Gi Label-based volume selection - Mark which volumes contribute to ephemeral storage Sum all non-root volumes

What's the recommended approach for this use case? How important is this feature to you?

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