Add instance storage discovery patterns in config

[Ivaylogi98]

Note: Should merge forward these changes to ubuntu-noble.

Summary

Adds two config fields to the AWS agent settings baked into the stemcell, required by
cloudfoundry/bosh-agent#407 (and subsequently by cloudfoundry/bosh-agent#396) to correctly discover NVMe instance storage on AWS Nitro
instances.

Background

On Nitro-based instances (e.g. m6id, i3en), NVMe PCIe enumeration order is
non-deterministic — /dev/nvme0n1 may be the root EBS volume, an attached EBS volume,
or instance storage depending on boot order. The bosh-agent now uses symlinks in
/dev/disk/by-id/ to identify and exclude EBS volumes, leaving only instance storage.

To keep the resolver IaaS-agnostic, the AWS-specific patterns are not hardcoded in the
agent binary — they are injected via agent config at stemcell build time.

Changes

stemcell_builder/stages/bosh_aws_agent_settings/apply.sh:

• InstanceStorageDevicePattern: glob pattern matching all NVMe namespace devices
• InstanceStorageManagedVolumePattern: glob pattern matching EBS volume symlinks in
  /dev/disk/by-id/, used to identify and exclude EBS volumes from instance storage
  discovery

Related

• Fix nvme instance storage discovery bosh-agent#407 — implements the NVMe instance storage discovery using
  these patterns
• Implement Runtime NVMe Instance Storage Discovery Using AWS EBS Symlinks bosh-agent#396 — original PR this refactors
• Fix NVMe raw_instance_storage device enumeration for all instance families bosh-aws-cpi-release#196 — companion CPI-side change

This repository uses a "Merge Forward" strategy

Changes should be made in the earliest applicable branch, and
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3. Merge ubuntu-<short_name> into merge-to-<next_short_name>
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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

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• bosh-stemcell/spec/stemcells/aws_spec.rb

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Walkthrough

The apply.sh emitter now adds two NVMe-related keys under Platform.Linux in the generated /var/vcap/bosh/agent.json: InstanceStorageDevicePattern set to /dev/nvmen1 and InstanceStorageManagedVolumePattern set to /dev/disk/by-id/nvme-Amazon_Elastic_Block_Store_. The existing UseMonitIptablesFirewall setting remains. A new RSpec context parses agent.json and asserts those two values.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main change: adding instance storage discovery patterns to the AWS agent configuration, which is the primary focus of the changeset.
Description check	✅ Passed	The pull request description is comprehensive and well-structured, covering summary, background, changes, and related items. It exceeds the template requirements by providing substantial context about the NVMe discovery problem and implementation approach.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@stemcell_builder/stages/bosh_aws_agent_settings/apply.sh`:
- Around line 15-16: The two config keys InstanceStorageDevicePattern and
InstanceStorageManagedVolumePattern are dead fields (not consumed by
bosh-agent); either remove them from the agent config output in apply.sh or
implement support in bosh-agent to consume them. To fix: if removing, delete the
two keys from the config generation in apply.sh and update any docs/tests; if
implementing, add parsing and use of
InstanceStorageDevicePattern/InstanceStorageManagedVolumePattern inside the
DevicePathResolver logic (respecting existing DevicePathResolutionType,
DIDTransformPattern and VolumeID matching semantics), add unit tests to
bosh-agent that verify matching/resolution against /dev/disk/by-id/ symlinks,
and update integration tests to exercise managed vs instance-storage resolution
paths.

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Fix all unresolved CodeRabbit comments on this PR:

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• Create a new PR with the fixes

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• stemcell_builder/stages/bosh_aws_agent_settings/apply.sh

[Copilot]

Pull request overview

Adds AWS-specific BOSH agent configuration needed for NVMe instance storage discovery on Nitro-based EC2 instances.

Changes:

• Adds a glob for NVMe namespace devices.
• Adds a glob for AWS EBS /dev/disk/by-id symlinks so managed volumes can be excluded from instance storage discovery.

---

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[aramprice]

Double check that new agent config won't break agents that don't yet know how to consume it before merging.

[neddp]

Hi @aramprice,

The new config fields themselves are safe to add to the stemcell: agent.json is loaded via a plain json.Unmarshal into a Go struct (app/config.go#L30). Per the official Go docs (pkg.go.dev/encoding/json):

"When parsing a JSON object into a Go struct, unknown keys in the JSON object are ignored (unless a Decoder is used and Decoder.DisallowUnknownFields has been called)."

So an older agent binary without InstanceStorageDevicePattern / InstanceStorageManagedVolumePattern in its LinuxOptions will simply skip them - no error, no behaviour change.

Note that an old agent + new CPI on NVMe instances is not a new regression. The old CPI was already sending unreliable NVMe paths due to non-deterministic PCIe enumeration order - NVMe instance storage was already broken before this change set. A partial deployment (new CPI only, old agent) is no worse than the pre-existing broken state. The full fix requires new CPI + new agent + this stemcell config, all deployed together.

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.

[aramprice]

/coderabbitai review

[neddp]

@coderabbitai review

[coderabbitai]

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.