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