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[sean mooney <2069192@xxxxxxxxxxxxxxxxxx>]

Public bug reported:

https://docs.openstack.org/nova/latest/configuration/config.html#libvirt.mem_stats_period_seconds

setting the mem_stats_period_seconds value to 0 is documented as
disabling the memory ballon

in practice it disable the generation of the generate of the

 <memballoon model='virtio'/> element.

in older version of libvirt that had the effect of disabling the mem
ballon

in libvirt 0.8.4 libvrt gaind the ability to disable the mem ballon
explicitly by setting


<memballoon model='none'/>

and at some later point libvirt started adding a mem ballon deivece by
default if one is not present in the XML

while the exact version of libvirt that change was made in is not known
the net effect is

[libvirt]
mem_stats_period_seconds=0 no longer disbales the memory balloon as documented.


mem_stats_period_seconds

    Type:

        integer
    Default:

        10

    A number of seconds to memory usage statistics period. Zero or
negative value mean to disable memory usage statistics.


disabling the memory balloon and statistics is important for minimising
the overhead of tracking that memory usage in realtime vms

since nova does not currently set

autodeflate the mem balloon in nova had no other use except metric reporting so when we set 
mem_stats_period_seconds=0  we should explicitly disable the device in the XML to restore the old behavior.

a future enhancement could be to set autodeflate=on when the device is
present.

this bug also interact with sev.

sev requires all virtio devices to have the iommu=on driver option set 
if you disabel mem stats today and libvirt adds a mem-ballon automatically it will not have this element set and that can cause issues with some feature like sev.

** Affects: nova
     Importance: Low
         Status: Triaged


** Tags: libvirt low-hanging-fruit realtime

** Tags added: low-hanging-fruit

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https://bugs.launchpad.net/bugs/2069192

Title:
  nova can nolonger disabel the mem ballon device

Status in OpenStack Compute (nova):
  Triaged

Bug description:
  https://docs.openstack.org/nova/latest/configuration/config.html#libvirt.mem_stats_period_seconds

  setting the mem_stats_period_seconds value to 0 is documented as
  disabling the memory ballon

  in practice it disable the generation of the generate of the

   <memballoon model='virtio'/> element.

  in older version of libvirt that had the effect of disabling the mem
  ballon

  in libvirt 0.8.4 libvrt gaind the ability to disable the mem ballon
  explicitly by setting

  
  <memballoon model='none'/>

  and at some later point libvirt started adding a mem ballon deivece by
  default if one is not present in the XML

  while the exact version of libvirt that change was made in is not
  known the net effect is

  [libvirt]
  mem_stats_period_seconds=0 no longer disbales the memory balloon as documented.


  
  mem_stats_period_seconds

      Type:

          integer
      Default:

          10

      A number of seconds to memory usage statistics period. Zero or
  negative value mean to disable memory usage statistics.


  disabling the memory balloon and statistics is important for
  minimising the overhead of tracking that memory usage in realtime vms

  since nova does not currently set

  autodeflate the mem balloon in nova had no other use except metric reporting so when we set 
  mem_stats_period_seconds=0  we should explicitly disable the device in the XML to restore the old behavior.

  a future enhancement could be to set autodeflate=on when the device is
  present.

  this bug also interact with sev.

  sev requires all virtio devices to have the iommu=on driver option set 
  if you disabel mem stats today and libvirt adds a mem-ballon automatically it will not have this element set and that can cause issues with some feature like sev.

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