We have two Dell PowerEdge R720 servers hosting only five VMs between them. We need two more VMs. I'm not sure...
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whether to trust the salesman who wants us to buy additional equipment. Do I need another VM host server?
Capacity estimation is a common scenario with virtualization, but every case is different.
Factors determining a VM host server's ability to increase its workload include available capacity and speeds of CPU, RAM and disk, along with some limitations depending on your host OS -- for example, VMware ESXi running version 5.1 had a 32 gigabyte (GB) memory limit for the entire physical server.
If you've already got the hardware in place and a few VMs running on it, you've got some pros and cons already. Your hardware options are rather inflexible, but at least you can benchmark the servers you have. Benchmarking tells you what capacity you're currently using and how much room there is for more. There's no way an external salesperson can know what your VMs are using without looking themselves, and even then they must make the estimate over a reasonable amount of time, incorporating peak times of usage as well as average. Ends of months are often a peak times for server utilization.
Also consider what would happen if a VM host server failed. If you only have two physical hosts, one single host may have to run all the VMs at once. Whether it's a disaster situation, routine updates and patches on the host or the cleaning person unplugging a server to vacuum, an outage can be avoided if you can live migrate all VMs to one server. In an ideal world, you'll have 50% of resources available at peak on each host to cover the whole load, but more likely you can get away with something a bit lower than peak resource usage. It's more important that services are running, but it may be OK that they are just a little slower.
With all this and potential time constraints in mind, there is an alternative to adding hardware. Build up the ones you need and see how it goes. It's definitely not a recommended approach, but if you're dealing with nonbusiness critical applications, then it's a quicker way to determine if the added VMs can function without additional hardware resources. At worst, you shut the new VMs down, buy more hardware and spin up your new servers on it.
About the author:
Adam Fowler is IT operations manager at a law firm in Australia. He's worked in IT for over a decade, including responsibilities in systems, infrastructure and operational service.
Editor's note: The asker provides their Dell PowerEdge R720 server specs as follows: Dual Intel Xeon 8Core Process 2.6GHz, 96 GB RAM, 6 -- 16GB RDIMM, 8 -- 600GB 15K RPM SAS.
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