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doc: add migration design documentation
This commit adds migration design doc which carry information about the required changes and design for rbd intree to csi migration. Fixes ceph#2596 Updates ceph#2509 Signed-off-by: Humble Chirammal <[email protected]>
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| # In-tree storage plugin to CSI Driver Migration | ||
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| Prior to CSI, Kubernetes provided a powerful volume plugin system. These volume | ||
| plugins were “in-tree” meaning their code was part of the core Kubernetes code | ||
| and shipped with the core Kubernetes binaries. The CSI migration effort enables | ||
| the replacement of existing in-tree storage plugins such as kubernetes.io/rbd | ||
| with a corresponding CSI driver. For more information on CSI migration effort | ||
| refer [design doc](https://github.com/kubernetes/community/blob/master/contributors/design-proposals/storage/csi-migration.md) | ||
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| ## RBD in-tree plugin to CSI migration | ||
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| This feature is tracked in Kubernetes for v1.23 as alpha under | ||
| [RBD in-tree migration KEP](https://github.com/kubernetes/enhancements/issues/2923) | ||
| . Below are the changes identified in the design discussions to enable CSI | ||
| migration for RBD plugin: | ||
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| ### ClusterID field in the migration request | ||
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| Considering the `clusterID` field is a required one for CSI, but in-tree | ||
| StorageClass has `monitors` field as a required thing the clusterID will be sent | ||
| from the migration library based on the monitors field, Kubernetes storage admin | ||
| supposed to create a clusterID based on the monitors hash ( ex: `#md5sum <<< | ||
| "<monaddress:port>"`) in the CSI config map and keep the monitors under this | ||
| configuration before enabling the migration. While CSI driver receive the volume | ||
| ID it will look at the configmap and figure out the monitors to do the | ||
| operations. Thus, CSI operations are unchanged or untouched wrt to the clusterID | ||
| field. | ||
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| ### New Volume ID for existing volume operations | ||
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| The existing volume will have a volume ID passed in similar to the following | ||
| format: | ||
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| ``` | ||
| mig_mons-<monitors-hash>_image-<imageUUID>_<pool-hash> | ||
| ``` | ||
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| Details on the hash values: | ||
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| * Monitors Hash: this carry a hash value (md5sum) which will be acted as the | ||
| `clusterID` for the operations in this context. | ||
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| * ImageUUID: this is the unique UUID generated by Kubernetes for the created | ||
| volume. | ||
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| * PoolHash: this is an encoded string of pool name. | ||
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| The existing in-tree volume's node operations should work without any issues as | ||
| those will be tracked as static volumes for the CSI driver. From above volume | ||
| ID, the CSI driver can also connect to the backend cluster and clean/delete the | ||
| image. The migration volume ID carry the information like monitors, pool and | ||
| image name which is good enough for the driver to identify and connect to the | ||
| backend cluster for its operations. With above volume ID in place, we will be | ||
| able to support mount, unmount, delete and resize operations. | ||
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| ### Migration secret for CSI operations | ||
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| The in-tree secret has a data field called `key` which is the base64 admin | ||
| secret key. The ceph CSI driver currently expect the secret to contain data | ||
| field `UserKey` for the equivalent. The CSI driver also expect the `UserID` | ||
| field which is not available in the in-tree secret by default. This | ||
| missing `userID` will be filled (if the username differ than `admin`) in the | ||
| migration secret as `adminId` field in the migration request, ie: | ||
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| `key` field value will be picked up from the migration secret to `UserKey` | ||
| field. | ||
| `adminId` field value will be picked up from the migration secret to `UserID` | ||
| field | ||
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| if `adminId` field is nil or not set, `UserID` field will be filled with default | ||
| value ie `admin`.The above logic get activated only when the secret is a | ||
| migration secret, otherwise skipped to the normal workflow as we have today. | ||
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| ## CephFS in-tree plugin to CSI migration | ||
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| This is yet to be done and tracked | ||
| at [CephFS in-tree migration KEP](https://github.com/kubernetes/enhancements/issues/2924) | ||
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| ### Additional Reference | ||
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| [Kubernetes 1.17 Feature: Kubernetes In-Tree to CSI Volume Migration Moves to Beta](https://kubernetes.io/blog/2019/12/09/kubernetes-1-17-feature-csi-migration-beta/) | ||
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| [Tracker Issue in Ceph CSI](https://github.com/ceph/ceph-csi/issues/2509) | ||
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| [In-tree storage plugin to CSI Driver Migration KEP](https://github.com/kubernetes/enhancements/issues/625) |