yahoo-eng-team team mailing list archive

[Jeroen Groenewegen van der Weijden <2112373@xxxxxxxxxxxxxxxxxx>]

Public bug reported:

When booting instances from volume, Nova ignores the flavor's hw:scsi_model extra spec and falls back to a default (e.g., lsilogic). This results in a mismatch between the expected and actual SCSI controller model used in the libvirt domain definition.
Problem Description

Nova allows defining SCSI controller behavior through hw:scsi_model and
hw:disk_bus in flavor extra_specs, which are respected when an instance
boots from an image. However, for volume-backed instances, the flavor’s
extra_specs are ignored, and Nova silently defaults to a hardcoded value
(typically lsilogic).

This inconsistency makes it difficult for operators to enforce a standard controller model (e.g., virtio-scsi) across all workloads, regardless of boot method.
Use Case

    Operator defines a flavor with hw:scsi_model=virtio-scsi and hw:disk_bus=scsi.
    Instance is booted from a volume (no image).
    Nova generates libvirt domain XML with SCSI controller model set to lsilogic instead of virtio-scsi.
    The mismatch causes degraded guest performance or failure to boot (e.g., UEFI Windows guests with incorrect controller).

Proposed Change
Ensure that Nova uses hw:scsi_model from the flavor's extra_specs when:
    The instance is booted from volume,
    No image metadata is available,
    No controller is explicitly defined.
This fallback logic should be added in _get_guest_storage_config() to set the controller model from the flavor's extra_specs if the image metadata does not define one.

Proposed Code Change
/usr/lib/python3/dist-packages/nova/virt/libvirt/driver.py

## org line
-        scsi_controller = self._get_scsi_controller(image_meta)
## added code starts here
+        scsi_controller = self._get_scsi_controller(
+            image_meta or objects.ImageMeta.from_dict({'properties': {}}))+
+
+        if not scsi_controller and flavor:
+            extra_specs = flavor.get('extra_specs', {}) if isinstance(flavor, dict) else flavor.extra_specs
+            scsi_model = extra_specs.get('hw:scsi_model', 'lsilogic')
+            scsi_controller = vconfig.LibvirtConfigGuestController()
+            scsi_controller.type = 'scsi'
+            scsi_controller.model = scsi_model
## added code stops here

** Affects: nova
     Importance: Undecided
         Status: New


** Tags: boot-from-volume flavor libvirt scsi

-- 
You received this bug notification because you are a member of Yahoo!
Engineering Team, which is subscribed to OpenStack Compute (nova).
https://bugs.launchpad.net/bugs/2112373

Title:
  Flavor Extra Specs for SCSI Controller Model Not Honored for Volume
  Boot

Status in OpenStack Compute (nova):
  New

Bug description:
  When booting instances from volume, Nova ignores the flavor's hw:scsi_model extra spec and falls back to a default (e.g., lsilogic). This results in a mismatch between the expected and actual SCSI controller model used in the libvirt domain definition.
  Problem Description

  Nova allows defining SCSI controller behavior through hw:scsi_model
  and hw:disk_bus in flavor extra_specs, which are respected when an
  instance boots from an image. However, for volume-backed instances,
  the flavor’s extra_specs are ignored, and Nova silently defaults to a
  hardcoded value (typically lsilogic).

  This inconsistency makes it difficult for operators to enforce a standard controller model (e.g., virtio-scsi) across all workloads, regardless of boot method.
  Use Case

      Operator defines a flavor with hw:scsi_model=virtio-scsi and hw:disk_bus=scsi.
      Instance is booted from a volume (no image).
      Nova generates libvirt domain XML with SCSI controller model set to lsilogic instead of virtio-scsi.
      The mismatch causes degraded guest performance or failure to boot (e.g., UEFI Windows guests with incorrect controller).

  Proposed Change
  Ensure that Nova uses hw:scsi_model from the flavor's extra_specs when:
      The instance is booted from volume,
      No image metadata is available,
      No controller is explicitly defined.
  This fallback logic should be added in _get_guest_storage_config() to set the controller model from the flavor's extra_specs if the image metadata does not define one.

  Proposed Code Change
  /usr/lib/python3/dist-packages/nova/virt/libvirt/driver.py

  ## org line
  -        scsi_controller = self._get_scsi_controller(image_meta)
  ## added code starts here
  +        scsi_controller = self._get_scsi_controller(
  +            image_meta or objects.ImageMeta.from_dict({'properties': {}}))+
  +
  +        if not scsi_controller and flavor:
  +            extra_specs = flavor.get('extra_specs', {}) if isinstance(flavor, dict) else flavor.extra_specs
  +            scsi_model = extra_specs.get('hw:scsi_model', 'lsilogic')
  +            scsi_controller = vconfig.LibvirtConfigGuestController()
  +            scsi_controller.type = 'scsi'
  +            scsi_controller.model = scsi_model
  ## added code stops here

To manage notifications about this bug go to:
https://bugs.launchpad.net/nova/+bug/2112373/+subscriptions