[ovirt-users] Cannot create VM with custom memory amount

[Gianluca Cecchi]

On Wed, Jan 11, 2017 at 6:31 AM, TranceWorldLogic . <
tranceworldlogic at gmail.com> wrote:

> Thanks Ondra that link is useful,
> Do you have any Idea why VDSM disable balloon concept for RHEV and use for
> all non-RHEV ?
>
> Please refer to below link.
> https://www.ovirt.org/documentation/sla/memory-balloon/
>
>
> *"Currently the balloon memory device is being created by default by
> libvirt for non RHEV environments, and VDSM disables it by default for
> RHEV."*
> Is there different technique use by RHEV ? Please provide me link for it
> detail.
> It will help me to compare ovirt and RHEV
> *.*
>
> Thanks,
> ~Rohit
>
>
In effect it is strange.
Inside official docs for RHEV 4.0 we have:
Administration Guide
https://access.redhat.com/documentation/en/red-hat-virtualization/4.0/paged/administration-guide/52-cluster-tasks

"
5.2.2.2. Optimization Settings Explained
Memory Balloon
Selecting the Enable Memory Balloon
Optimization check box enables memory
overcommitment on virtual machines running
on the hosts in this cluster. When this option
is set, the Memory Overcommit Manager
(MoM) will start ballooning where and when
possible, with a limitation of the guaranteed
memory size of every virtual machine.
...
It is important to understand that in some
scenarios ballooning may collide with KSM. In
such cases MoM will try to adjust the balloon
size to minimize collisions. Additionally, in
some scenarios ballooning may cause sub-
optimal performance for a virtual machine.
Administrators are advised to use ballooning
optimization with caution.
"

So it seems it is indeed possible to enable ballooning (note the
considerations about KSM that could partially answer to your question...)

But then in Virtual Machine Management Guide here
https://access.redhat.com/documentation/en/red-hat-virtualization/4.0/paged/virtual-machine-management-guide/33-installing-guest-agents-and-drivers

I see
"
3.3.1. Red Hat Virtualization Guest Agents and Drivers

virtio-balloon

Virtio-balloon is used to
control the amount of
memory a guest actually
accesses. It offers improved
memory over-commitment.
The balloon drivers are
installed for future
compatibility but not used by
default in Red Hat
Virtualization.
"

So it seems it is not possible to enable it for guests....
Any clarification?

Thanks,
Gianluca
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