quorum even node number

[pvps1]

Hi

is it a problem having an even node number (4,6,8,....) for quorum?
we have a 4 node cluster where nodes are rebooting erratic (when very high load I guess...)

if yes, is incrementing quorum_votes: 1 to quorum_votes: 2 on one node the solution?

regards
Peter

 

[wolfgang]

Hi,

even node numbers are no problem.
if you have 4 nodes you can have 1 node offline without any problems.

Changing the votes is no good idea because you give one node more weight then other ones.

 

[Alessandro 123]

So, any number of nodes greater than 3 is totally safe? What would happens in a 4 nodes cluster if 2 nodes are down ? You loose the quorum. The same for 6 nodes: 3 up, 3 down.

 

[mir]

You will always need majority among nodes. The formula for number of running nodes to achieve quorum is: floor((n/2)+1) for n > 0.

 

[Alessandro 123]

Yes but with an even number of node there is the very remote possibility that half cluster goes down (power failures or so on)
With an odd number this is not possible.

This could be solved by placing a single node with less redundancy level then the majority of nodes. In example: one PSU and no redundant network.

 

[mir]

I don't get your point. Why is the chance for loosing more than one node at the same time higher with a cluster of even nodes compared to a cluster with uneven nodes?

 

[Alessandro 123]

It depends on the server model. If you have one PSU and 20 nodes, 10 connected to feed1, 10 connected to feed2, if you loose feed1, you loose half cluster, thus, no quorum.

 

[Alessandro 123]

One question: in a 20 nodes cluster, if I loose 11 nodes, I also loose quorum, right ?

 

[mir]

It depends on the server model. If you have one PSU and 20 nodes, 10 connected to feed1, 10 connected to feed2, if you loose feed1, you loose half cluster, thus, no quorum.

Is this particular setup, yes.

 

[mir]

One question: in a 20 nodes cluster, if I loose 11 nodes, I also loose quorum, right ?

floor((20/2)+1) = 11. So to have quorum you need 11 running nodes so quorum is lost if not running nodes > 9.

 

[Alessandro 123]

floor((20/2)+1) = 11. So to have quorum you need 11 running nodes so quorum is lost if not running nodes > 9.

Ok, so my previous thinking is totally non-sense.