Consistency groups

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A consistency group is a set of volumes that can be used to create a point in time snapshots, or to create a consistency copy of volumes. PowerVC consistency groups feature uses OpenStack generic volume group APIs.

Users with Administrator or Storage manager role can create consistency groups, and group snapshots.

For creating consistency group, a group type must exist. Group type is used to categorize a group just as storage template is used to describe the volume properties. However, they are common across multiple storage drivers. Post installation, PowerVC creates the default group types, which can be used for consistency group creation.
  • default_grouptype: Used to create consistency group and consistency group snapshots. The consistent_group_snapshot_enabled property is enabled for this group type.
  • default_grouptype_with_replication: Used with volume with replication properties. The consistent_group_replication_enabled property, which allows consistency group creation for replicated volumes, is enabled for this group type.
  • default_group_type_grs: Default group type for Global Replication Services API, used internally by PowerVC for APIs. GUI support is not available for this group type.

You can create additional group types from the GUI. PowerVC also has an additional group type "replication_enabled_vmrm. Use this group type when using PowerVC with VM Recovery Manager for IBM Storwize® volumes in Global Mirror relationship Global mirror with VM Recovery Manager (VMRM) for High Availability (HA).

Working with consistency groups

Consider the following points when you work with consistency groups.
  • Volume can be part of only a single consistency group.
  • Consistency group can be created only for volumes from a single storage provider.
  • You can create consistency group for both available and in-use volumes.
  • Volume with replication enabled must use the replication group type.
  • On successful creation of consistency group, it can be used to create Group Snapshot. Group snapshot creates volume snapshots for volumes, which are part of a snapshot group.
  • You can also create consistency group, by selecting an existing consistency group. To do this use PowerVC Clone icon. Cloning a consistency group is only allowed for consistency group with volumes in available state. If consistency group has an in-use volume, then create a group snapshot and then use that for creating consistency group.
  • You can add or remove volumes from a consistency group. Maximum number of volumes that can added in consistency group depends on the storage driver specification.
    Note: PowerMax does not allow to add or remove volumes from a consistency group, if a group snapshot exists.
  • When a group snapshot is deleted, it also deletes the associated volume snapshots.
  • When the consistency group is deleted, it also deletes the volumes, which are part of the consistency group. Before deleting a consistency group ensure that there are no associated group snapshots.
  • If a virtual machine or a volume with consistency group or a consistency group snapshot is unmanaged from PowerVC, the volume entry is removed from the PowerVC cinder database but they remain in the storage backend. If the same virtual machine or volume is remanaged in PowerVC the consistency group and group snapshot information is lost.
  • In SAN Volume Controller (SVC), if the number of volumes added in a group exceeds the maximum limit, the group enters an error state. You can reset the group status on the user interface to restore availability.

Consistency group created outside of PowerVC

PowerVC does not support out of band (OOB) operations on consistency groups for the volumes that are managed within PowerVC. If the consistency group is created outside of PowerVC for the volumes that are managed within PowerVC, then some of the volume lifecycle operations are impacted.

OOB behaviors
  • The OOB consistency group is not visible in the PowerVC GUI under the consistency groups page. For FlashSystem if the volume is a part of the consistency group, then the volume is visible under the volume metadata properties.
  • There is no impact for volume attach or detach operations.
  • Volume resize is not allowed if the volume is a part of the consistency group.
  • Virtual machine or volume delete operation from PowerVC fails if the volume is a part of the consistency group. Remove the volume from consistency group manually while you perform delete operations.
  • Volume retype operation fails if the volume is a part of the consistency group.

Limitations

  • Consistency group is not supported for Shared Storage Pools, and GPFS.
  • Consistency group of in-use volumes is not supported in DS8K.
  • For PowerMax, consistency group for SRDF volumes is not supported.
  • Renaming of PowerMax groups is not allowed.
  • The following operations are not allowed if a volume is in a consistency group.
    • Volume retype or migration
    • Volume delete
    • Clone volumes
    • Virtual machine snapshot for the volumes in a consistency group.
    • Live migration