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# Provisioning and mounting CephFS snapshot-backed volumes
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Snapshot-backed volumes allow CephFS subvolume snapshots to be exposed as
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regular read-only PVCs. No data cloning is performed and provisioning such
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volumes is done in constant time.
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For more details please refer to [Snapshots as shallow read-only volumes](./design/proposals/cephfs-snapshot-shallow-ro-vol.md)
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design document.
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## Prerequisites
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Prerequisites for this feature are the same as for creating PVCs with snapshot
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volume source. See [Create snapshot and Clone Volume](./snap-clone.md) for more
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information.
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## Usage
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### Provisioning a snapshot-backed volume from a volume snapshot
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For provisioning new snapshot-backed volumes, following configuration must be
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set for storage class(es) and their PVCs respectively:
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* StorageClass:
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* Specify `backingSnapshot: "true"` parameter.
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* PersistentVolumeClaim:
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* Set `storageClassName` to point to your storage class with backing
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snapshots enabled.
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* Define `spec.dataSource` for your desired source volume snapshot.
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* Set `spec.accessModes` to `ReadOnlyMany`. This is the only access mode that
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is supported by this feature.
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### Mounting snapshots from pre-provisioned volumes
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Steps for defining a PersistentVolume and PersistentVolumeClaim for
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pre-provisioned CephFS subvolumes are identical to those described in
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[Static PVC with ceph-csi](./static-pvc.md), except one additional parameter
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must be specified: `backingSnapshotID`. CephFS-CSI driver will retrieve the
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snapshot identified by the given ID from within the specified subvolume, and
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expose it to workloads in read-only mode. Volume access mode must be set to
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`ReadOnlyMany`.
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Note that the snapshot retrieval is done by traversing `<rootPath>/.snap` and
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searching for a directory that contains `backingSnapshotID` value in its name.
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The specified snapshot ID does not necessarily need to be the complete directory
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name inside `<rootPath>/.snap`, however it must be complete enough to uniquely
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identify that directory.
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Example:
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```
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$ ls .snap
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_f279df14-6729-4342-b82f-166f45204233_1099511628283
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_a364870e-6729-4342-b82f-166f45204233_1099635085072
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```
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`f279df14-6729-4342-b82f-166f45204233` would be considered a valid value for
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`backingSnapshotID` volume parameter, whereas `6729-4342-b82f-166f45204233`
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would not, as it would be ambiguous.
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If the given snapshot ID is ambiguous, or no such snapshot is found, mounting
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the PVC will fail with INVALID_ARGUMENT error code.
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