Persistent memory virtualization for virtual machine

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Persistent memory virtualization is a PowerVM® feature that allows you to create persistent memory volumes by using Dynamic Random Access Memory (DRAM) modules within IBM POWER9 and later servers.

Persistent memory enables data that is stored in memory to persist even after an operating system is shutdown. This feature reduces startup time. This support provides reduced downtime for planned and unplanned maintenance outages of the partition. However, this feature does not provide any benefit after a Consumer Electronics Control (CEC) reboot.

Prerequisites
  • The virtual machine must be in shutoff state for add or remove persistent memory volume operations.
  • The virtual machine can be in a shutoff or active state when the persistent memory volume is renamed.
  • No other task must be performed on the virtual machine.
Supported use cases
PowerVC supports the following use case scenarios:
  • Deploy a virtual machine with one or more persistent volumes by using the Create server API or the Deploy page in the GUI.
  • Create a compute template with one or more persistent volumes and capacity.
  • View, add, or remove persistent volumes of an existing virtual machine.
  • Add or remove persistent memory volumes of an existing virtual machine and retain the aggregate memory used. Aggregate memory refers to the sum of regular RAM and the total size of the allocated persistent memory volumes.
  • Rename a persistent memory volume
  • Migration and remote restart operations on the virtual machine with persistent memory volume for hosts with FW 1060 or later.
  • Onboard a virtual machine with persistent volumes.
Considerations
  • PowerVC allows only DRAM-based persistent memory volumes.
  • Persistent memory volume is not supported for IBM i virtual machines.
  • PHYP currently does not support resizing persistent memory volumes. However, the persistent memory volumes can be deleted and new persistent memory volumes can be created with the required size.
  • The maximum number of persistent memory volumes per virtual machine is restricted to four by PHYP. Therefore, a validation during deployment and add persistent memory volume is implemented to ensure that this limit is adhered to.
  • Persistent memory volume has the following statuses:
    • CONFIGURING - When a request to add a persistent memory volume is received, it enters the CONFIGURING state.
    • ACTIVE - The persistent memory volume moves to the ACTIVE state after it is successfully added to the virtual machine.
    • ERROR - If the add or delete operation of a persistent memory volume fails, the volume moves to the ERROR state.
    • DEGRADED - If the delete operation of a persistent memory fails and the persistent memory volumes still exists in the backend, the volume moves to DEGRADED state in PowerVC. You can retry the delete operation of the persistent memory volume to remove it from the backend and from the virtual machine.
  • The persistent memory volume name is limited to 20 characters. Also, each name must be unique within a workspace.
  • The maximum number of persistent memory volumes on a host is limited to 62 or 64, depending on the host type or configuration.
  • If persistent memory volumes are created out-of-band (OOB), or when you manage an OOB virtual machine with persistent memory volumes, ensure that each volume has a unique name to enable smooth mobility operations. Otherwise, mobility operations might fail due to name conflicts.
  • If the deployment of the virtual machine succeeds but the addition of persistent memory volumes fails, the overall deployment process is not marked as failed. Instead, only the persistent memory volume creation step fails, and the volumes enter an error state with the corresponding status code. The persistent memory volumes that are in an error state also appear in the GET response of the server. You can try to add the persistent memory volumes again.