[ClusterLabs] Antw: [EXT] Re: Live migration possible with KSM ?

Ulrich Windl Ulrich.Windl at rz.uni-regensburg.de
Tue Apr 6 03:34:36 EDT 2021

>>> "Lentes, Bernd" <bernd.lentes at helmholtz-muenchen.de> schrieb am 31.03.2021 um
15:58 in Nachricht
<537587707.120787942.1617199127012.JavaMail.zimbra at helmholtz-muenchen.de>:

> ----- On Mar 30, 2021, at 7:54 PM, hunter86 bg hunter86_bg at yahoo.com wrote:
>> Keep in mind that KSM is highly cpu intensive and is most suitable for same 
> type
>> of VMs,so similar memory pages will be merged until a change happen (and 
> that
>> change is allocated elsewhere).
>> In oVirt migration is possible with KSM actively working, so it should work 
> with
>> pacemaker.
>> I doubt that KSM would be a problem... most probably performance would not 
> be
>> optimal.
>> Best Regards,
>> Strahil Nikolov
>>> On Tue, Mar 30, 2021 at 19:47, Andrei Borzenkov
>>> <arvidjaar at gmail.com> wrote:
>>> On 30.03.2021 18:16, Lentes, Bernd wrote:
>>> > Hi,
>>>> currently i'm reading "Mastering KVM Virtualization", published by Packt
>>> > Publishing, a book i can really recommend.
>>>> There are some proposals for tuning guests. One is KSM (kernel samepage
>>> > merging), which sounds quite interesting.
>>>> Especially in a system with lots of virtual machines with the same OS this 
> could
>>> > lead to significant merory saving.
>>>> I'd like to test, but i don't know if KSM maybe prevents live migration in a
>>> > pacemaker cluster.
>>> I do not think pacemaker cares or is aware about KSM. It just tells
>>> resource agent to perform migration; what happens is entirely up to
>>> resource agent.
>>> If you can migrate without pacemaker you can also migrate with pacemaker.
> Just to give a feedback.
> I configured KSM on both nodes. On one it saves me nearly 20GB RAM.
> I checked live migration and it worked.

It' like with any sharing: If you need to "unshare", then you need extra memory. So I wouldn't use that "extra" 20GB to run more VMs.

> Bernd

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