I'd like to throw a quick question out there.
My company has 645 VMs spread over 13 ESXi 4.1 hosts (avg 49 VMs per host).
To improve performance they are suggesting I create 3 seperate Clusters and split the 13 ESXi Hosts among them. (i.e. 4 Hosts for Dev / 4 hosts for Pre-Prod and 5 hosts for Production VMs). I don't think this is will have any performance benefit whatsoever, in fact as we dont have an equal number of dev, pre-prod and prod vms we could be in a situation of worse performance as more prod servers would be on fewer hosts than present.
All Hosts are IBM servers (64 Logical CPUs, 400GB RAM)
Personally I feel 1 Cluster is fine, and we simply need to add more Hosts to gain more CPU & RAM and have less VMs on Hosts -.i.e. less load.
What do you guys think? Is there any benefit to having multiple Clusters? Any personal experience would be good to know.
Duncan wrote a nice article about cluster sizes with vSphere 5.0 (with a lot of interessting comments):
If you would create 3 clusters keep in mind that you need to provide 3 failover hosts minimum. In your 13 node cluster you are may fine with a 12 + 1 setup but with 3 cluster setup it must be three times n + 1.
Are you seeing performance issues currently? A cluster of 13 hosts isn't all that large, and you can probably resolve any issues you're seeing by using things like Network I/O Control, Storage I/O Control, Resource Pools/Shares, etc instead of splitting up clusters.
Thanks for the reply. Currently in the performance view of our single Cluster I see this:-
I think the rationale behind multiple clusters is to get more out of our servers (so says the PM) - but I dont think this is right.
Interesting. Would you be able to elaborate on their thoughts on how splitting up clusters gets more out of the systems? As vMario pointed out, it's the opposite.
In answer to the question, No we arent really seeing too much in the way of performance related issues. The project wants to create some large VMs (large in CPU & RAM) and think this is the best way to manage those. I'd thought a single cluster would be better? Am I wrong?
I think all they will achieve is have 3 administrative 'islands'. This is typical PM / Business Owner thinking by the way.
If you don´t have any problems I don´t see a reason to split up the cluster.
One main reason often are licensing facts. Oracle I think is the most common use case where spliting up the cluster make sense by fact saving much licensing costs.
I can understand their idea, but they are sacrificing resources (due to HA needs) in order to split it up. I would keep it one cluster unless there is a proven reason to do otherwise. Perhaps others will have different thoughts.
there are a number of reasons to split clusters the link to Duncans blog highlight them from a technical view point, if you are not using Blade technology then I would not bother spliting.
Another major reason is license compliance, as the other poster says, this is usually for a Oracle Cluster.
by spliting up your environment you will be reducing your overall resources as you will need to introduce more failover hosts, currently you have two hosts assigned for failover purposes, if you split into three seperate Clusters then there will be a requriement for an extra host to be assigned as a failover host.
overall this seem a waste of resource.
you say that your environment is looking at larger VM's this is just a capacity planning exercise, I would continue your arguement, as you are on the right lines.
egg3 & Tom - thank you very much for your input here. I appreciate it!
Our licensing is ok at the moment - I just think there is some skewed thinking in the business around this. They actually pinched one of my Hosts a while ago to use as a Citrix XenApp server - maybe I should ask for it back !
I would most definately ask for it back and virtualise it
Apart from other benefits highlighted above, you also need to take into account that more hosts will give DRS more flexibility to load balance the cluster.
In our 4.1 environment I feel 16 hosts cluster is an ideal number and have capped it to that.
one additional discussion for your reading pleasure