Add Volume Snapshot Support

[xing-yang]

This document proposes to add Kubernetes snapshot support in-tree.

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[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: xing-yang
We suggest the following additional approver: saad-ali

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[liggitt]

I was expecting the cloud provider-related API and controller to be out of tree. Isn't the ideal scenario that any storage provider would be able to support snapshotting their storage, interacting with the core kubernetes API only via VolumeSnapshot/VolumeSnapshotData objects?

[liggitt]

to that end, it would probably make sense for there to be a place for opaque, storage-provider-specific parameters in the VolumeSnapshotData, so the storage provider can persist the info required to locate the snapshot (similar to the parameters in StorageClass and FlexVolumeSource)

[xing-yang]

Thanks @liggitt for the comments. This design is modeled after the design of PersistentVolume and PersistentVolumeClaim. For example, PersistentVolumeSource contains storage provider data as follows:

type PersistentVolumeSource struct {
// GCEPersistentDisk represents a GCE Disk resource that is attached to a
// kubelet's host machine and then exposed to the pod.
// +optional
GCEPersistentDisk *GCEPersistentDiskVolumeSource

// AWSElasticBlockStore represents an AWS EBS disk that is attached to a
// kubelet's host machine and then exposed to the pod
// +optional
AWSElasticBlockStore *AWSElasticBlockStoreVolumeSource
...
}

Having opaque, storage-provider-specific parameters in the VolumeSnapshotData is an option we can discuss about.

[mtanino]

/ok-to-test

[kmova]

Most of the storage vendors already support volume level snapshots. It will be good to include a section on how these will be made available via the CSI drivers.

[ScorpioCPH]

nit: extra white space between [Xing Yang] and link.

[xing-yang]

Will address it. Thanks.

[xing-yang]

@kmova Sure, will add a section for that later.

[xing-yang]

Since I re-signed CLA with a new email address, I need to close this PR and open a new PR to get the CLA issue resolved. Thanks.

[cblecker]

@xing-yang You shouldn't need to. You should also be able to follow directions like https://stackoverflow.com/questions/3042437/change-commit-author-at-one-specific-commit to reset the author on your commits to match your e-mail address

[xing-yang]

@cblecker Thanks. I tried that in my other PR's but couldn't resolve it until I re-submit a new one:(. I have got help from Linux Foundation help desk as well.

Here's the new PR with CLA issue resolved: #1662