✨ Add EphemeralDisk to InstanceSpec

[EmilienM]

What this PR does / why we need it:

An additional block device can be attached to the instance. It won't be bootable and only optional.
It can be added aside of a RootVolume or not.

apiVersion: infrastructure.cluster.x-k8s.io/v1alpha7
kind: OpenStackMachineTemplate
metadata:
  name: <cluster-name>-controlplane
  namespace: <cluster-name>
spec:
...
  ephemeralDisk:
     format: "ext4"
     size: 10
...

The local disk will be deleted when the instance is removed.

A use case is when having etcd running on machines that boot from volumes, you can now have a dedicated local disk with higher I/O for it.

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[k8s-ci-robot]

Hi @EmilienM. Thanks for your PR.

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: EmilienM
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[EmilienM]

This is just a prototype for now. Also for some reasons I can't make the conversion working yet.

[pierreprinetti]

/ok-to-test

[EmilienM]

/uncc seanschneeweiss jichenjc

[EmilienM]

/cc pierreprinetti mdbooth dulek

[EmilienM]

struggling a bit with the conversion & their tests...

[ching-kuo]

You might need to restore the value during up conversion like this PR #1517

[jichenjc]

for new spec I suggest create an issue to track the proposed change and create release notes for it... @EmilienM

[mdbooth]

Couple of things to think about:

There's prior art here: https://github.com/kubernetes-sigs/cluster-api-provider-azure/blob/e7773fe017f3493be65c3368808d726a6343911e/api/v1beta1/azuremachine_types.go#L77-L82

It might be worth considering using the same terminology unless there's a good reason to do otherwise.

Also consider: how do I specify that I want a secondary disk and it's the nova local disk, vs it's a cinder disk? I suppose it might be neater to give the local disk a separate field in the API because there can only be one of them, vs any number of cinder disks.

I believe the CAPZ folks have done the whole thing end-to-end, i.e. they're also able to mount the disk. Might be worth looking into how they did that.

[mdbooth]

It occurs to me that to use a local disk as a data disk it could just be a flag. I'm not sure we need to specify the size of the local disk. We can just take whatever we're given by the flavor. It is possible to specify a smaller disk than the one the flavor gave you, but:

• We would probably be the only people using this feature, so we'd be the ones finding all the bugs¹
• IIRC from an accounting POV Nova still assumes you used the whole disk, so you might as well use it

If we added to machine spec something like LocalDiskIsData (or something better: that's a terrible name), then:

• It would be an error if there was no root volume
• We'd construct an explicit BDM² with the local disk second
• Later we could add a separate field for DataDisks provided by Cinder, which could be added to the BDM after the local disk.

Footnotes

1. I used to be a maintainer of this code ↩

2. Block Device Mapping ↩

[EmilienM]

@mdbooth thanks, I'm looking into this
@jichenjc I'll do it, once we settle on the API needs, based on Matt's comments. I'll come up with something and create an issue.

[EmilienM]

/hold

[k8s-ci-robot]

PR needs rebase.

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