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[OpenStack Infra <1835926@xxxxxxxxxxxxxxxxxx>]

This issue was fixed in the openstack/nova queens-eol  release.

** Changed in: nova/queens
       Status: Fix Committed => Fix Released

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https://bugs.launchpad.net/bugs/1835926

Title:
  Volume attachment may fail after rescuing instance on an image with
  different hw_disk_bus

Status in OpenStack Compute (nova):
  Fix Released
Status in OpenStack Compute (nova) queens series:
  Fix Released
Status in OpenStack Compute (nova) rocky series:
  Fix Committed
Status in OpenStack Compute (nova) stein series:
  Fix Committed
Status in OpenStack Compute (nova) train series:
  Fix Released

Bug description:
  Description
  ===========

  Look likes rescue may update instances.root_device_name if rescue image has different disk bus (image property hw_disk_bus) than instance.
  This introduce a mimatch between device name and driver used for instance:

  During instance config generation, nova guess the disk bus driver according table instance_system_metadata.image_hw_disk_bus,
  and get root device name from table instances.root_device_name.
  Because of this mismatch, cinder attachment may failed with the following error message in compute log:
   unable to execute QEMU command 'device_add': Duplicate ID 'virtio-disk0' for device

  Probable solution is to avoid rescue action to update
  instance.root_device_name

  Steps to reproduce
  ==================

  On a fresh master devstack:
  openstack image save cirros-0.4.0-x86_64-disk --file /tmp/cirros-0.4.0-x86_64-disk.disk
  #create a new image, but an scsi one:
  openstack image create --container-format bare --disk-format qcow2  --file /tmp/cirros-0.4.0-x86_64-disk.disk --property hw_disk_bus='scsi' --property  hw_scsi_model='virtio-scsi' cirros-0.4.0-x86_64-scsi-disk
  #create instance with default virtio driver:
  openstack server create  --flavor m1.small --image cirros-0.4.0-x86_64-disk --nic net-id=private test
  mysql> select root_device_name from instances where uuid='xxx'
  /dev/vda
  #rescue instance but with the scsi image: 
  $openstack server rescue xxxx --image  cirros-0.4.0-x86_64-scsi-disk
  mysql> select root_device_name from instances where uuid='xxx'
  /dev/sda
  $openstack server unrescue xxxx
  # root_device_name is still on sda should be on vda according instance metadata
  mysql> select root_device_name from instances where uuid='xxx'
  /dev/sda
  $virsh dumpxml instance-00000001 | grep "bus='virtio"
  	  <target dev='vda' bus='virtio'/>
  	  <alias name='virtio-disk0'/>
  # at the next hard reboot new xml is generated with scsi device name BUT with virtio driver.
  $openstack server reboot --hard xxx
  $virsh dumpxml instance-00000001 | grep -A 1 "bus='virtio"
  	  <target dev='sda' bus='virtio'/>
  	  <alias name='virtio-disk0'/>
  $openstack volume create --size 10 test
  $openstack server add volume 1c9b1582-5fc7-417a-a8a0-387e8833731f 0621430c-b0d2-4cca-8868-f86f36f1ef29
  $sudo journalctl -u devstack@n-cpu.service | grep Duplicate
  Jul 05 09:29:54 alex-devstack-compute2 nova-compute[28285]: ERROR nova.virt.libvirt.driver [None req-38714989-4deb-4a05-bdfc-3418edbda7e3 demo demo] [instance: 1c9b1582-5fc7-417a-a8a0-387e8833731f] Failed to attach volume at mountpoint: /dev/vda: libvirtError: internal error: unable to execute QEMU command 'device_add': Duplicate ID 'virtio-disk0' for device

  Error probably comes from the fact that nova lookup for next availiable virtio device based on name, which is  vda - virtio-disk0 (as root device is currently sda)
  but because root device sda is already using virtio-disk0 it failed.

  
  Expected result
  ===============
  instance root_device_name should remain the same as before rescue/unrescue, regardless of image used for rescuing.

  
  Actual result
  =============
  instance root_device_name is updated according the hw_disk_bus property for the image used during rescue(and never set back to original value)

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