# Attaching virtual persistent memory to guests

Added in version 20.0.0: (Train)

Starting in the 20.0.0 (Train) release, the virtual persistent memory (vPMEM) feature in Nova allows a deployment using the libvirt compute driver to provide vPMEMs for instances using physical persistent memory (PMEM) that can provide virtual devices.

PMEM must be partitioned into PMEM namespaces for applications to use. This vPMEM feature only uses PMEM namespaces in `devdax` mode as QEMU vPMEM backends. If you want to dive into related notions, the document NVDIMM Linux kernel document is recommended.

To enable vPMEMs, follow the steps below.

## Dependencies

The following are required to support the vPMEM feature:

- Persistent Memory Hardware

  One such product is Intel® Optane™ DC Persistent Memory. ipmctl is used to configure it.

- Linux Kernel version >= 4.18 with the following modules loaded:

  `dax_pmem`, `nd_pmem`, `device_dax`, `nd_btt`

Note

NVDIMM support is present in the Linux Kernel v4.0 or newer. It is recommended to use Kernel version 4.2 or later since NVDIMM support is enabled by default. We met some bugs in older versions, and we have done all verification works with OpenStack on 4.18 version, so 4.18 version and newer will probably guarantee its functionality.

- QEMU version >= 3.1.0

- Libvirt version >= 5.0.0

- ndctl version >= 62

- daxio version >= 1.6

The vPMEM feature has been verified under the software and hardware listed above.

## Configure PMEM namespaces (Compute)

1. Create PMEM namespaces as vPMEM backends using the ndctl utility.

   For example, to create a 30GiB namespace named `ns3`:

       $ sudo ndctl create-namespace -s 30G -m devdax -M mem -n ns3
       {
         "dev":"namespace1.0",
         "mode":"devdax",
         "map":"mem",
         "size":"30.00 GiB (32.21 GB)",
         "uuid":"937e9269-512b-4f65-9ac6-b74b61075c11",
         "raw_uuid":"17760832-a062-4aef-9d3b-95ea32038066",
         "daxregion":{
           "id":1,
           "size":"30.00 GiB (32.21 GB)",
           "align":2097152,
           "devices":[
           {
             "chardev":"dax1.0",
             "size":"30.00 GiB (32.21 GB)"
           }
           ]
         },
         "name":"ns3",
         "numa_node":1
       }

   Then list the available PMEM namespaces on the host:

       $ ndctl list -X
       [
         {
           ...
           "size":6440353792,
           ...
           "name":"ns0",
           ...
         },
         {
           ...
           "size":6440353792,
           ...
           "name":"ns1",
           ...
         },
         {
           ...
           "size":6440353792,
           ...
           "name":"ns2",
           ...
         },
         {
           ...
           "size":32210157568,
           ...
           "name":"ns3",
           ...
         }
       ]

2. Specify which PMEM namespaces should be available to instances.

   Edit `libvirt.pmem_namespaces`:

       [libvirt]
       # pmem_namespaces=$LABEL:$NSNAME[|$NSNAME][,$LABEL:$NSNAME[|$NSNAME]]
       pmem_namespaces = 6GB:ns0|ns1|ns2,LARGE:ns3

   Configured PMEM namespaces must have already been created on the host as described above. The conf syntax allows the admin to associate one or more namespace `$NSNAME`s with an arbitrary `$LABEL` that can subsequently be used in a flavor to request one of those namespaces. It is recommended, but not required, for namespaces under a single `$LABEL` to be the same size.

3. Restart the `nova-compute` service.

   Nova will invoke ndctl to identify the configured PMEM namespaces, and report vPMEM resources to placement.

## Configure a flavor

Specify a comma-separated list of the `$LABEL`s from `libvirt.pmem_namespaces` to the flavor’s `hw:pmem` property. Note that multiple instances of the same label are permitted:

    $ openstack flavor set --property hw:pmem='6GB' my_flavor
    $ openstack flavor set --property hw:pmem='6GB,LARGE' my_flavor_large
    $ openstack flavor set --property hw:pmem='6GB,6GB' m1.medium

Note

If a NUMA topology is specified, all vPMEM devices will be put on guest NUMA node 0; otherwise nova will generate one NUMA node automatically for the guest.

Based on the above examples, an `openstack server create` request with `my_flavor_large` will spawn an instance with two vPMEMs. One, corresponding to the `LARGE` label, will be `ns3`; the other, corresponding to the `6G` label, will be arbitrarily chosen from `ns0`, `ns1`, or `ns2`.

Note

Using vPMEM inside a virtual machine requires the following:

- Guest kernel version 4.18 or higher;

- The `dax_pmem`, `nd_pmem`, `device_dax`, and `nd_btt` kernel modules;

- The ndctl utility.

Note

When resizing an instance with vPMEMs, the vPMEM data won’t be migrated.

## Verify inventories and allocations

This section describes how to check that:

- vPMEM inventories were created correctly in placement, validating the configuration described above.

- allocations were created correctly in placement for instances spawned from flavors configured with vPMEMs.

Note

Inventories and allocations related to vPMEM resource classes are on the root resource provider related to the compute node.

1. Get the list of resource providers

       $ openstack resource provider list
       +--------------------------------------+--------+------------+
       | uuid                                 | name   | generation |
       +--------------------------------------+--------+------------+
       | 1bc545f9-891f-4930-ab2b-88a56078f4be | host-1 |         47 |
       | 7d994aef-680d-43d4-9325-a67c807e648e | host-2 |         67 |
       --------------------------------------+---------+------------+

2. Check the inventory of each resource provider to see resource classes

   Each `$LABEL` configured in `libvirt.pmem_namespaces` is used to generate a resource class named `CUSTOM_PMEM_NAMESPACE_$LABEL`. Nova will report to Placement the number of vPMEM namespaces configured for each `$LABEL`. For example, assuming `host-1` was configured as described above:

       $ openstack resource provider inventory list 1bc545f9-891f-4930-ab2b-88a56078f4be
       +-----------------------------+------------------+----------+----------+-----------+----------+--------+
       | resource_class              | allocation_ratio | max_unit | reserved | step_size | min_unit |  total |
       +-----------------------------+------------------+----------+----------+-----------+----------+--------+
       | VCPU                        |             16.0 |       64 |        0 |         1 |        1 |     64 |
       | MEMORY_MB                   |              1.5 |   190604 |      512 |         1 |        1 | 190604 |
       | CUSTOM_PMEM_NAMESPACE_LARGE |              1.0 |        1 |        0 |         1 |        1 |      1 |
       | CUSTOM_PMEM_NAMESPACE_6GB   |              1.0 |        3 |        0 |         1 |        1 |      3 |
       | DISK_GB                     |              1.0 |      439 |        0 |         1 |        1 |    439 |
       +-----------------------------+------------------+----------+----------+-----------+----------+--------+

   Here you can see the vPMEM resource classes prefixed with `CUSTOM_PMEM_NAMESPACE_`. The `LARGE` label was configured with one namespace (`ns3`), so it has an inventory of `1`. Since the `6GB` label was configured with three namespaces (`ns0`, `ns1`, and `ns2`), the `CUSTOM_PMEM_NAMESPACE_6GB` inventory has a `total` and `max_unit` of `3`.

3. Check allocations for each server that is using vPMEMs

       $ openstack server list
       +--------------------------------------+----------------------+--------+-------------------+---------------+-----------------+
       | ID                                   | Name                 | Status | Networks          | Image         | Flavor          |
       +--------------------------------------+----------------------+--------+-------------------+---------------+-----------------+
       | 41d3e139-de5c-40fd-9d82-016b72f2ba1d | server-with-2-vpmems | ACTIVE | private=10.0.0.24 | ubuntu-bionic | my_flavor_large |
       | a616a7f6-b285-4adf-a885-dd8426dd9e6a | server-with-1-vpmem  | ACTIVE | private=10.0.0.13 | ubuntu-bionic | my_flavor       |
       +--------------------------------------+----------------------+--------+-------------------+---------------+-----------------+

       $ openstack resource provider allocation show 41d3e139-de5c-40fd-9d82-016b72f2ba1d
       +--------------------------------------+------------+------------------------------------------------------------------------------------------------------------------------+
       | resource_provider                    | generation | resources                                                                                                              |
       +--------------------------------------+------------+------------------------------------------------------------------------------------------------------------------------+
       | 1bc545f9-891f-4930-ab2b-88a56078f4be |         49 | {u'MEMORY_MB': 32768, u'VCPU': 16, u'DISK_GB': 20, u'CUSTOM_PMEM_NAMESPACE_6GB': 1, u'CUSTOM_PMEM_NAMESPACE_LARGE': 1} |
       +--------------------------------------+------------+------------------------------------------------------------------------------------------------------------------------+

       $ openstack resource provider allocation show a616a7f6-b285-4adf-a885-dd8426dd9e6a
       +--------------------------------------+------------+-----------------------------------------------------------------------------------+
       | resource_provider                    | generation | resources                                                                         |
       +--------------------------------------+------------+-----------------------------------------------------------------------------------+
       | 1bc545f9-891f-4930-ab2b-88a56078f4be |         49 | {u'MEMORY_MB': 8192, u'VCPU': 8, u'DISK_GB': 20, u'CUSTOM_PMEM_NAMESPACE_6GB': 1} |
       +--------------------------------------+------------+-----------------------------------------------------------------------------------+

   In this example, two servers were created. `server-with-2-vpmems` used `my_flavor_large` asking for one `6GB` vPMEM and one `LARGE` vPMEM. `server-with-1-vpmem` used `my_flavor` asking for a single `6GB` vPMEM.

this page last updated: 2026-08-20 01:33:50

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