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- Fixes spelling mistakes. - Grammatical error correction. - Wrapping the text at 80 line count..etc Signed-off-by: Humble Chirammal <hchiramm@redhat.com>
102 lines
4.6 KiB
Markdown
102 lines
4.6 KiB
Markdown
# RBD MIRRORING
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RBD mirroring is a process of replication of RBD images between two or more Ceph
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clusters. Mirroring ensures point-in-time, crash-consistent RBD images between
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clusters, RBD mirroring is mainly used for disaster recovery (i.e. having a
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secondary site as a failover).
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See [Ceph documentation](https://docs.ceph.com/en/latest/rbd/rbd-mirroring) on
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RBD mirroring for complete information.
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## Architecture
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## Design
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Currently, CEPH-CSI generates its unique ID for each RBD image and stores the
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corresponding PersistentVolume (PV) name and the unique ID mapping, It creates
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the RBD image with the unique ID and returns the encoded value which contains
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all the required information for other operations. For mirroring, the same RBD
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image will be mirrored to the secondary cluster. As the journal(OMAP data) is
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not mirrored to the secondary cluster, The RBD images corresponding to the PV
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can not be identified without OMAP data.
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**Pre-req** It's expected that the Kubernetes Admin/User will create the static
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PersistentVolumeClaim (PVC) on the secondary site during the failover.
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**Note:** when the static PVC created on the secondary site we cannot use the
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VolumeHandle to identify the OMAP data nor the image anymore because as we have
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only PoolID and ClusterID in the VolumeHandle. We cannot identify the correct
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pool name from the PoolID because pool name will remain the same on both
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clusters but not the PoolID even the ClusterID can be different on the secondary
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cluster.
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> Sample PV spec which will be used by rbdplugin controller to regenerate OMAP
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> data
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```yaml
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csi:
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controllerExpandSecretRef:
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name: rook-csi-rbd-provisioner
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namespace: rook-ceph
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driver: rook-ceph.rbd.csi.ceph.com
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fsType: ext4
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nodeStageSecretRef:
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name: rook-csi-rbd-node
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namespace: rook-ceph
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volumeAttributes:
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clusterID: rook-ceph
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imageFeatures: layering
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imageFormat: "2"
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imageName: csi-vol-0c23de1c-18fb-11eb-a903-0242ac110005
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journalPool: replicapool
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pool: replicapool
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radosNamespace: ""
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volumeHandle: 0001-0009-rook-ceph-0000000000000002-0c23de1c-18fb-11eb-a903-0242ac110005
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```
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> **VolumeHandle** is the unique volume name returned by the CSI volume plugin’s
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> CreateVolume to refer to the volume on all subsequent calls.
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Once the static PVC is created on the secondary cluster, the Kubernetes User can
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try to delete the PVC,expand the PVC or mount the PVC. In case of mounting
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(NodeStageVolume) we will get the volume context in RPC call but not in the
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Delete/Expand Request. In Delete/Expand RPC request only the VolumeHandle
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(`clusterID-poolID-volumeuniqueID`) will be sent where it contains the encoded
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information related to ClusterID and PoolID. The VolumeHandle is not useful in
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secondary cluster as the PoolID and ClusterID always may not be the same.
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> In this design document we will talk about new controller(rbdplugin
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> controller) not replication controller, in next releases we will design the
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> replication controller to perform mirroring operations. The rbdplugin
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> controller will run as a sidecar in RBD provisioner pod.
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To solve this problem, We will have a new controller(rbdplugin controller)
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running as part of provisioner pod which watches for the PV objects. When a PV
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is created it will extract the required information from the PV spec, and it
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will regenerate the OMAP data. Whenever Ceph-CSI gets a RPC request with older
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VolumeHandle, it will check if any new VolumeHandle exists for the old
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VolumeHandle. If yes, it uses the new VolumeHandle for internal operations (to
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get pool name, Ceph monitor details from the ClusterID etc).
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Currently, We are making use of watchers in node stage request to make sure
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ReadWriteOnce (RWO) PVC is mounted on a single node at a given point in time. We
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need to change the watchers logic in the node stage request as when we enable
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the RBD mirroring on an image, a watcher will be added on a RBD image by the rbd
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mirroring daemon.
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To solve the ClusterID problem, If the ClusterID is different on the second
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cluster, the admin has to create a new ConfigMap for the mapped ClusterID's.
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Whenever Ceph-CSI gets a request, it will check if the ClusterID mapping exists
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and uses the mapped ClusterID to get the information like Ceph monitors etc.
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Details about creation of clusterID's mapping are available at
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[clusterID-mapping](./clusterid-mapping.md).
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**This design does not cover the below items:**
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* Bootstrapping RBD mirror daemon.
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* Mirroring of PVC snapshots
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* Mirroring for topology provisioned PVC.
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* Documenting the steps to handle failover/fallback of an image.
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* Workflow of a Replication controller.
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