[ovirt-users] Hot Memory add and Physical Memory guaranteed

[Luca 'remix_tj' Lorenzetto]

On Fri, Jun 23, 2017 at 11:16 AM, Milan Zamazal <mzamazal at redhat.com> wrote:
> "Luca 'remix_tj' Lorenzetto" <lorenzetto.luca at gmail.com> writes:
>
>> i just tested the memory hot add to a vm. This vm had 2048 MB. I set
>> the new memory to 2662 MB.
>> I logged into the vm and i've seen that hasn't been any memory change,
>> even if i said to the manager to apply memory expansion immediately.
>>
>> Memory shown by free -m is 1772 MB.
>
> [...]
>
>> forgot to say that is a RHEL 7 VM and has the memory baloon device enabled.
>
> This is normal with memory balloon enabled – memory balloon often
> "consumes" the hot plugged memory, so you can't see it.

Ok.

What's exactly the role of "guaranteed memory"? Is only about ensuring
on startup time that there is at least X free memory on hosts or
something more complex?

What's the best configuration? keeping baloon or not? setting memory
and guaranteed memory to the same value?
If i have 1TB of ram in all the cluster, does the "guaranteed memory"
doesn't allows to provision vms with cumulative guaranteed memory
usage greater than 1TB?
Can KSM help allow to overprovision in this situation?

Does memory baloon device has impacts on vm performance?

I need to understand better in order to plan correctly all the
required hardware resource i need for migrating to oVirt.

Luca

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Luca 'remix_tj' Lorenzetto, http://www.remixtj.net , <lorenzetto.luca at gmail.com>