** Changed in: nova
     Assignee: Matt Riedemann (mriedem) => Eric M Gonzalez (egrh3)

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

** Changed in: nova/queens
       Status: New => Confirmed

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https://bugs.launchpad.net/bugs/1731986

Title:
  nova snapshot_volume_backed failure does not thaw filesystems

Status in OpenStack Compute (nova):
  In Progress
Status in OpenStack Compute (nova) ocata series:
  Confirmed
Status in OpenStack Compute (nova) pike series:
  Confirmed
Status in OpenStack Compute (nova) queens series:
  Confirmed

Bug description:
  Noticed in OpenStack Mitaka (commit 9825c80), but the function
  (snapshot_volume_backed) is unchanged as of commit a4fc1bcd. backends:
  Libvirt + Ceph.

  When Nova attempts to create an image / snapshot of a volume-backed
  instance it first quiesces the instance in `snapshot_volume_backed()`.
  It then loops over all of the block devices associated with that
  instance. However, there is no exception handling in the for loop and
  any failures on the part of Cinder are bubbled up and through the
  `snapshot_volume_backed()` function. This causes the needed
  `unquiesce()` to never be called on the instance, leaving it in an
  inconsistent (read-only) state. This can cause operational errors in
  the instance leaving it unusable.

  In my case, the steps for reproduction are:

  1) nova create image / ( "create snapshot" via horizon )
  2) nova/compute/api snapshot_volume_backed() calls quiesce
  3) "qemu-ga: info: guest-fsfreeze called" is seen in instance
  4) cinder fails snapshot of volume due to OverLimit
  5) cinder raises OverLimit
  6) snapshot_volume_backed() never finishes due to OverLimit
  7) filesystem is never thawed
  8) instance unusable

  I am in the process of writing and testing a patch and will have a
  review for it soon.

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