The CSI external-snapshotter is part of Kubernetes implementation of Container Storage Interface (CSI).

The volume snapshot feature supports CSI v1.0 and it has been an Alpha feature in Kubernetes since v1.12.

CSI Snapshotter is an external controller that watches Kubernetes Snapshot CRD objects and triggers CreateSnapshot/DeleteSnapshot against a CSI endpoint. Full design can be found at Kubernetes proposal at here

External snapshotter follows controller pattern and uses informers to watch for VolumeSnapshot and VolumeSnapshotContent create/update/delete events. It filters out these objects with Snapshotter==<CSI driver name> specified in the associated VolumeSnapshotClass object and then processes these events in workqueues with exponential backoff.

Snapshotter talks to CSI over socket (/run/csi/socket by default, configurable by -csi-address). The snapshotter then:

• Discovers the supported snapshotter name by GetDriverName call.

• Uses ControllerGetCapabilities for find out if CSI driver supports ControllerServiceCapability_RPC_CREATE_DELETE_SNAPSHOT and ControllerServiceCapability_RPC_LIST_SNAPSHOTS calls. Otherwise, the controller will not start.

• Processes new/updated/deleted VolumeSnapshots: The snapshotter only processes VolumeSnapshot that has snapshotter specified in its VolumeSnapshotClass matches its driver name. The process workflow is as follows

  • If the snapshot status is Ready, the controller checks whether the snapshot and its content still binds correctly. If there is any problem with the binding (e.g., snapshot points to a non-exist snapshot content), update the snapshot status and emit event.
  • If the snapshot status is not ready, there are two cases.
    • SnapshotContentName is not empty: the controller verifies whether the snapshot content exists and also binds to the snapshot. If verification passes, the controller binds the snapshot and its content objects and marks it is ready. Otherwise, it updates the error status of the snapshot.
    • SnapshotContentName is set empty: the controller will first check whether there is already a content object which binds the snapshot correctly with snapshot uid (VolumeSnapshotRef.UID) specified. If so, the controller binds these two objects. Otherwise, the controller issues a create snapshot operation. Please note that if the error status shows that snapshot creation already failed before, it will not try to create snapshot again.

• Processes new/updated/deleted VolumeSnapshotContents: The snapshotter only processes VolumeSnapshotContent in which the CSI driver specified in the spec matches the controller's driver name.

  • If the VolumeSnapshotRef is set to nil, skip this content since it is not bound to any snapshot object.
  • Otherwise, the controller verifies whether the content object is correctly bound to a snapshot object. In case the VolumeSnapshotRef.UID is set but it does not match its snapshot object or snapshot no long exists, the content object and its associated snapshot will be deleted.

For debugging, it is possible to run snapshotter on command line. For example,

$ csi-snapshotter -kubeconfig ~/.kube/config -v 5 -csi-address /run/csi/socket

It is necessary to create a new service account and give it enough privileges to run the snapshotter. We provide .yaml files that deploy for use together with the hostpath example driver. A real production deployment must customize them:

$ for i in $(find deploy/kubernetes -name '*.yaml'); do kubectl create -f $i; done

Running Unit Tests:

$ go test -timeout 30s  github.com/kubernetes-csi/external-snapshotter/pkg/controller

$ dep ensure

To modify dependencies or versions change ./Gopkg.toml

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