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

[Martin Sivak]

> 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?

As Milan said, it tells the balloon the lower limit of ballooning. We
won't take more memory from the VM when it reaches this amount of
assigned memory. It is not related to VM start, but to runtime.

It is important to realize that both overcommit features (KSM,
ballooning) require that the full VM can be started first. Only then
will KSM start deduplicating pages and balloon inflating.

That is why we recommend swap to be present and enabled on a host
although it won't be really used, it just provides a buffer space
before the VM is compacted.

> What's the best configuration? keeping baloon or not? setting memory
> and guaranteed memory to the same value?

Setting the two fields to the same value effectively disables
ballooning for the VM.

> 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?

No, the memory field itself behaves like this. You can allow the
consumption go higher by defining memory overcommit ratio (in Edit
Cluster dialog). Guaranteed memory is only used for controlling
balloon limits.

> Can KSM help allow to overprovision in this situation?

KSM is complementary to ballooning in this case. Both features allow
over commit to work.

- KSM needs no cooperation of the VM, but scanning for duplicate pages
loads the host
- ballooning needs cooperation of the VM (driver + guest agent)

> Does memory baloon device has impacts on vm performance?

Well the device itself does not impose any load. But when you take
some memory from the VM, it will have obviously less resources. If the
VM tends to allocate huge amount of memory in short time then the
balloon might be slow in returning the memory to the VM and the
application might not be happy about it.


Best regards

Martin Sivak

On Fri, Jun 23, 2017 at 11:58 AM, Luca 'remix_tj' Lorenzetto
<lorenzetto.luca at gmail.com> wrote:
> 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
>
> --
> "E' assurdo impiegare gli uomini di intelligenza eccellente per fare
> calcoli che potrebbero essere affidati a chiunque se si usassero delle
> macchine"
> Gottfried Wilhelm von Leibnitz, Filosofo e Matematico (1646-1716)
>
> "Internet è la più grande biblioteca del mondo.
> Ma il problema è che i libri sono tutti sparsi sul pavimento"
> John Allen Paulos, Matematico (1945-vivente)
>
> Luca 'remix_tj' Lorenzetto, http://www.remixtj.net , <lorenzetto.luca at gmail.com>
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