Merge "Add nested resource providers limit for multi create"

api-guide/source/accelerator-support.rst

@@ -10,8 +10,9 @@ To launch servers with accelerators, the administrator (or an user with
 appropriate privileges) must do the following:
 
 * Create a device profile in Cyborg, which specifies what accelerator
-  resources need to be provisioned. (See `Cyborg device profiles API
+  resources need to be provisioned. (See `Cyborg device profiles API`_.)
-  <https://docs.openstack.org/api-ref/accelerator/v2/index.html#device-profiles>`_.
+
+  .. _`Cyborg device profiles API`: https://docs.openstack.org/api-ref/accelerator/v2/index.html#device-profiles
 
 * Set the device profile name as an extra spec in a chosen flavor,
   with this syntax:
@@ -54,3 +55,28 @@ below:
 
 Some operations, such as lock and unlock, work as they are effectively
 no-ops for accelerators.
+
+Caveats
+-------
+
+.. note::
+
+   This information is correct as of the 21.0.0 Ussuri release. Where
+   improvements have been made or issues fixed, they are noted per item.
+
+For nested resource providers:
+
+* Creating servers with accelerators provisioned with the Cyborg service, if
+  a flavor asks for resources that are provided by nested Resource Provider
+  inventories (e.g. vGPU) and the user wants multi-create (i.e. say --max 2)
+  then the scheduler could be returning a NoValidHosts exception even if each
+  nested Resource Provider can support at least one specific instance, if the
+  total wanted capacity is not supported by only one nested Resource Provider.
+  (See `bug 1874664 <https://bugs.launchpad.net/nova/+bug/1874664>`_.)
+
+  For example, creating servers with accelerators provisioned with the Cyborg
+  service, if two children RPs have 4 vGPU inventories each:
+
+    - You can ask for a device profile in the flavor with 2 vGPU with --max 2.
+    - But you can't ask for a device profile in the flavor with 4 vGPU and
+      --max 2.

doc/source/admin/virtual-gpu.rst

@@ -385,6 +385,26 @@ For libvirt:
 
      This has been resolved in the Rocky release. See `bug 1762688`_.
 
+For nested vGPUs:
+
+.. note::
+
+   This information is correct as of the 21.0.0 Ussuri release. Where
+   improvements have been made or issues fixed, they are noted per item.
+
+* If creating servers with a flavor asking for vGPUs and the user wants
+  multi-create (i.e. say --max 2) then the scheduler could be returning
+  a NoValidHosts exception even if each physical GPU can support at least
+  one specific instance, if the total wanted capacity is not supported by
+  only one physical GPU.
+  (See `bug 1874664 <https://bugs.launchpad.net/nova/+bug/1874664>`_.)
+
+  For example, creating servers with a flavor asking for vGPUs, if two
+  children RPs have 4 vGPU inventories each:
+
+    - You can ask for a flavor with 2 vGPU with --max 2.
+    - But you can't ask for a flavor with 4 vGPU and --max 2.
+
 For XenServer:
 
 * Suspend and live migration with vGPUs attached depends on support from the