Considerations of working with consistency groups and snapshots

This topic introduces the considerations when working with consistency group and snapshots.

Common Considerations

  • A consistency group is empty upon its creation. You can add volumes to it after the creation.

  • You can add only volumes from a single storage provider to a consistency group.

  • You can create group snapshots from a consistency group (CG). A group snapshot is composed of multiple volume snapshots, one per volume that is contained in the CG. All the volume snapshots within a group snapshot are created at the same time.

  • You can add or remove volumes from a consistency group. The maximum number of volumes that can be added in a consistency group depends on the specification of the corresponding storage provider.

  • When a consistency group snapshot is deleted, it also deletes the associated volume snapshots.

  • A consistency group (CG) can support more than one storage templates that can be configured only when you create the CG. The supported storage templates of an existing CG are not allowed for edition. Only volumes that are created from the supported storage templates can be added to the CG.

    • When you create a CG on a storage provider (SP) from the UI of the IBM Cloud Infrastructure Center, you are not allowed to specify the supported storage templates for the CG. The CG supports all the existing storage templates of the SP.

    • When you create a CG on a storage provider (SP) from the API of the IBM Cloud Infrastructure Center, you can select more than one storage templates from the SP for the CG. The CG supports only the selected storage templates.

  • Cloning a consistency group (CG) is allowed if all the volumes in the CG are in Available state. If a CG has any volume that is in In-Use state, you can create a group snapshot from the CG, and then create a new CG from the group snapshot.

  • The data that is not flushed from a virtual machine's memory to the corresponding volumes, is not stored by creating a group snapshot from the consistency group that contains those volumes. If you want to store the in-memory data of an application by a group snapshot, you must flush those data to the volumes before you create the group snapshot.

  • Volume deletion is not allowed if the volume is in a consistency group.

  • Volume snapshot deletion is not allowed if the snapshot is a part of a group snapshot.

  • Consistency group deletion is not allowed if the group has snapshots or the group has volumes in In-Use state.

Consistency Group Considerations for IBM Storage FlashSystem

Consistency Group Considerations for IBM Storage DS8000

  • Before you create a consistency group, refer to Configuring LSS for consistency group to configure LSS first.

  • When you add volumes to a consistency group or create a consistency group from a source, and all the existing LSSs that are in the configured LSS range are either full or unavailable, a new LSS that is in the configured LSS range is created.

  • The volumes in In-use state cannot be added or removed to or from a consistency group.

  • In some scenarios, after you add or remove a volume to or from a consistency group, the IBM Cloud Infrastructure Center migrates the volume to a different LSS or storage pool so that the WWN, the LSS and the storage pool of the volume can be changed. Refer to Planning for working with consistency group for more description.

Consistency Group Considerations for IBM Storage Scale

  • The creation of a consistency group from snapshot is not supported.

  • To clone a consistency group is not supported.