PVE 8.3.5 Clustering and HA

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wifi_guy

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Jan 22, 2025
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Hi,

I am running a HA cluster across 3 identical servers. I understand for HA and quorum purposes you always need an odd number of servers.

Can I add a 4th and just not include it in my HA setup? Could I add this 4th server to the cluster for migration purposes and etc? My 4th server isn't identical and I mainly use it for a pre-production test environment before I rolling things onto my production servers.

Thoughts?

Thanks in advance.
 
wifi_guy said:
Can I add a 4th and just not include it in my HA setup?

Thoughts?
Click to expand...
Yes, you can ignore the 4th node for HA purposes. But that is not the problem! PVE needs Quorum to be established to work as intended. Without Quorum you can not administrate a cluster. This fact is completely independent from activating HA or not.

With three nodes you can lose one node without getting into trouble. (You already know this.)

With four nodes you can lose one node without getting into trouble. Nothing changes, in this regard!

But now there is a chance to create a split brain with two nodes in one group and two nodes in the other. This situation has to be avoided. That's why only one node can be lost within a four node cluster.

A dedicated Quorum Device is cheap. And it allows to lose two nodes without getting into trouble. If you are willing to run four nodes then a QD for a fifth vote is the logical recommendation. You can establish a QD a) on another dedicated hardware device like a Raspberry Pi (or better) b) as a VM on another (off-cluster-) system c) as a container or VM on a NAS or similar.

Good luck :-)
 
So what you are saying is by adding this 4th server to the cluster, even though I am not giving it an HA configuration, that server would automatically have voting rights.

Poop, OK. It would be a neat feature in PVE to be able to specify which servers are given quorum rights for voting. Have 10 servers in a cluster, maybe only 7 of the 10 have an HA configuration on them. That 7 you would specify "you are a voting node in this HA setup".

I think I am going to leave my 4th server on its own, backup my VM's that I need to transfer and do it that way. Migrating is so much cooler. Oh well.
 
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wifi_guy said:
So what you are saying is by adding this 4th server to the cluster, even though I am not giving it an HA configuration, that server would automatically have voting rights?
Click to expand...
Yes! As soon as it is added to the cluster as a member it gets one vote :)

The resources it does offer may be completely different in regards of Ram/Cpu/Disk-number/Disk-space etc. Theses technical differences will hinder migration if they do not fit, but as a member it still has a vote.
 
It would be nice to have the ability to add a server to a cluster and be able to choose if it gets voting rights or not. There are many benefits to have all servers in a cluster. Management being one of them. Migrating VMs another.
 
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Gotcha.

Let me ask you this. In my case, I have groups setup. For example, if PVE1 goes down, those VM's are going to PVE2. If 2 goes down both PVE 1 and 2 are going to 3.

Group Order
PVE1 > PVE2 > PVE3
PVE2 > PVE1 > PVE3
PVE3 > PVE2 > PVE1
 
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