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

• 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 $NSNAMEs 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 $LABELs 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

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.

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.

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.