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

[Milan Zamazal]

"Luca 'remix_tj' Lorenzetto" <lorenzetto.luca at gmail.com> writes:

> 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?

I think it defines the minimum memory that the balloon should always
leave available.

Martin, do you know answers to the other questions?

> 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