[Bug 1678275] Re: [SRU] cinder rbd calls block eventlet threads

[Matt Rae]

** Tags removed: sts-sponsor

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https://bugs.launchpad.net/bugs/1678275

Title:
  [SRU] cinder rbd calls block eventlet threads

Status in Ubuntu Cloud Archive:
  New

Bug description:
  [Impact]

  cinder-volume's rbd driver makes a call out to rbd it does not yield
  to eventlet, thus blocking all other processing. When this happens any
  pending requests are stuck unacknowledged in the rabbit queue until
  the current rbd task completes. This results in an unresponsive cloud
  presented to the user and actions such as instance creation failing
  due to nova timing out waiting on cinder.

  [Test Case]

  Steps to reproduce:
  1: Create a volume that will take more than an instant to delete.
  2: Delete the volume
  3: Immediately attempt to create some volumes

  Expected results:
  Volumes create in a timely manner and become available
  Volume delete processes and delete finishes in parallel

  [Regression Potential]

  This patch moves all rados calls to a separate python thread which
  +doesn't block eventlet loop.

  [Original Description LP: #1401335]

  When cinder-volume's rbd driver makes a call out to rbd it does not yield to eventlet, thus blocking all other processing.
  When this happens any pending requests are stuck unacknowledged in the rabbit queue until the current rbd task completes. This results in an unresponsive cloud presented to the user and actions such as instance creation failing due to nova timing out waiting on cinder.

  Requirements to reproduce:
  1: Ceph set up with a rbd backend
  2: A single ceph-volume worker to prevent the distributed nature from masking the problem
  3: A method of creating a large volume, writing to it

  Steps to verify volume will trigger issue on delete:
  1: Get the UUID of the volume you have created and dirtied
  2: Use the rbd command on your ceph cluster to delete the volume and verify it takes a couple minutes to delete.
  3: Delete the volume in cinder to cleanup cinder's database.

  Steps to reproduce:
  1: Create a volume that will take more than an instant to delete.
  2: Delete the volume
  3: Immediately attempt to create some volumes

  Expected results:
  Volumes create in a timely manner and become available
  Volume delete processes and delete finishes in parallel

  Actual results:
  Volumes creations are processed after the delete has finished
  Volume delete blocks threads and must process first

  As RBD commands consume a fair amount of CPU time to process we should not just background the RBD commands as that would represent a DoS risk for the cinder-volume hosts.
  One possible way to fix this would be to implement at least 2 queues that control the spawning of threads, reserving x of y threads for time sensitive and fast tasks.

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