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

Klaus Wenninger kwenning at redhat.com
Tue Apr 6 04:47:45 EDT 2021

On 4/6/21 9:34 AM, Ulrich Windl wrote:
>>>> "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.
Yep - probably safer to use it for giving the OS some extra room to
breathe with some cache-memory ;-)
>> Bernd
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