[openstack-dev] Resize behavior with smaller disk across hypervisors

Michael J Fork mjfork at us.ibm.com
Fri Apr 12 21:13:35 UTC 2013

While digging into https://bugs.launchpad.net/nova/+bug/1163844 concerning
the Hyper-V driver, I noticed the behavior when the disk shrinks is
inconsistent across all the hypervisors.

libvirt: logs it cannot resize to smaller and just keeps the larger disk

xen: tries to copy contents to a smaller disk, fails if too large

hyperv: always errors out if new disk size is smaller

powervm: silently keeps the larger disk

vmware: (couldn't find where migration code handled resize)

The only mention in the API guide -
  - is "scaling the server up or down."

What is the * expected * behavior here?  For metering reasons, my thought
is that if the disk cannot be sized down an error should be thrown and that
the libvirt and powervm should be modified.  One issue with this approach
is how we expose to the end-user what happened, I don't believe the ERROR
state has any details.  Additionally, auto-reverting the resize has the
same side effect.  I would like to explore standardizing the behavior in
Havana, documenting in the API guide, and sync'ing in the hyperivsors.



Michael Fork
Architect, OpenStack Development
IBM Systems & Technology Group
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