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hosts: make it clear what to expect and how to configure balloon feature on KVM #337

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yadvr merged 1 commit into from
Aug 4, 2023
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hosts: make it clear what to expect and how to configure balloon feature on KVM #337

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42 changes: 38 additions & 4 deletions source/adminguide/hosts.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -377,10 +377,11 @@ VMware tools installed.
KVM
'''

All VMs are required to support the virtio drivers. These drivers are
All KVM VMs are required to support the virtio drivers. These drivers are
installed in all Linux kernel versions 2.6.25 and greater. The
administrator must set CONFIG\_VIRTIO\_BALLOON=y in the virtio
configuration.
configuration. Drivers for Windows can be downloaded from
https://github.com/virtio-win/virtio-win-pkg-scripts


Hypervisor capability
Expand All @@ -396,12 +397,45 @@ The DMC (Dynamic Memory Control) capability of the hypervisor should be
enabled. Only XenServer Advanced and above versions have this feature.


VMware, KVM
'''''''''''
VMware
''''''

Memory ballooning is supported by default.


KVM
'''

Memory ballooning is supported and enabled by default. This can be configured on
per KVM host basis via the `vm.memballoon.disable=false` property and the
`vm.memballoon.stats.period` property in the `agent.properties` of the KVM host.

The memory ballooning feature on KVM allows the host to reclaim memory from
guest VMs which is enabled by a virtio balloon device on the guest VM and the
related virtio drivers inside the guest VMs. This feature is mainly intended to
support over-committing memory on KVM hosts.

A related feature, KSM (Kernel Same-page Merging), can also be enabled to assist
with over-committing memory. On some distributions such as Ubuntu, this is
enabled by default, and can be checked otherwise by checking/setting
`/sys/kernel/mm/ksm/run` to 1 and for libvirt set the `KSM_ENABLED=AUTO` in
`/etc/defaults/qemu-kvm`.

Note: the memory ballooning feature isn't automatic on KVM and shouldn't be
confused with the dynamic scaling feature that allows manual scaling of running
VMs by changing the service offering (feature can be enabled via the setting
enable.dynamic.scale.vm) of VMs that aren't using a fixed compute offering.

By default, when memory is over provisioned (setting mem.overprovisioning.factor
is greater than 1.0 at global or cluster level) the actual memory for the VM is
the memory per the service offering divided by the global or cluster-specific
memory overprovisioning factor. This means guests start with a lower memory than
their service offering intended memory, which will not be changed or scaled
automatically. When overcommitting memory, this behaviour can be disabled by
turning off (set the value false) the setting
`vm.min.memory.equals.memory.divided.by.mem.overprovisioning.factor`.


Setting Over-Provisioning Factors
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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