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[Matt Riedemann <mriedem@xxxxxxxxxx>]

** Also affects: nova/newton
   Importance: Undecided
       Status: New

** Changed in: nova/newton
       Status: New => In Progress

** Changed in: nova/newton
   Importance: Undecided => Medium

** Changed in: nova/newton
     Assignee: (unassigned) => Lee Yarwood (lyarwood)

** Changed in: nova
     Assignee: Feodor Tersin (ftersin) => Chris Friesen (cbf123)

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https://bugs.launchpad.net/bugs/1605720

Title:
  backing store missing for ephemeral disk on cold migration

Status in OpenStack Compute (nova):
  Fix Released
Status in OpenStack Compute (nova) newton series:
  In Progress

Bug description:
  I'm on stable/mitaka, but the master code looks similar.

  I have compute nodes configured to use qcow2 and libvirt.  The flavor
  has an ephemeral disk and a swap disk, with root disk size of "0" for
  auto-size.  I boot an instance with this flavor.

  When I try to cold-migrate the instance, I get an error:
  2016-07-21 23:33:48.561 46340 ERROR nova.compute.manager [instance: 4e52bfd8-0c71-48dc-89fb-6f6b31dc06bb] libvirtError: Cannot access backing file '/etc/nova/instances/_base/ephemeral_1_0706d66' of storage file '/etc/nova/instances/4e52bfd8-0c71-48dc-89fb-6f6b31dc06bb/disk.eph0' (as uid:0, gid:0): No such file or directory

  The problem seems to be that in
  nova.virt.libvirt.driver.LibvirtDriver.finish_migration() we call
  self._create_image(...block_device_info=None...)

  Down in _create_image() we handle the case of a "disk.local" ephemeral
  device, but that doesn't help because the device is actually named
  "disk.eph0".   It looks like we then try to loop over any ephemerals
  in block_device_info, but that's None so we don't handle any of those
  (which is too bad since it looks like they would be named correctly).

  The end result is that we have a qcow2 "disk.eph0" image, but with
  potentially no backing store in <instances_path>/_base.  When we tell
  libvirt to start the instance, this results in the above error.

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