Apr 04, 2011

How many virtual machines can I run on one system?

And what other factors do I need to consider?


The hard-and-fast limit to the number of VMs you can run is your computer's memory. However, the more virtual machines you have, the slower everything will get - in particular, disk I/O is typically the first and worst performance bottleneck you'll face, since most disk performance degrades much worse than linearly with increased concurrency.

Well server stuff aside, if it's a desktop system then it'll depend on your hardware. If you just mean storing them then it'll depend on your hard disk space. I have tons of Linux distros I run in VMs and it's really just a matter of space.

If you mean running them all at the same time then that's a different matter entirely.

Back to the server stuff, here's a brief article that covers your question:



"How many user desktops can run on a virtualized server? From a structural point of view, this question has a lot in common with, "How many angels can dance on the head of a pin?" Nevertheless, I did my best to answer it in the new McGraw-Hill book, Management Strategies for the Cloud Revolution. As it turns out, I was wrong.In trying to convey the looming power of the cloud over end user computing, I attempted to address the question of how many end users might be served a virtual desktop from a remote point if only one server were available. The answer I came up with was based on a four-way server with each CPU equipped with four cores, a rather common configuration these days. I said such a machine based on Nehalem cores could satisfy the needs of 256 ends users at a time, no problem. In effect, a server built with basic PC parts can match what used to be 256 individual PC desktops."

In Hyperv no of VM's depends on your Hardware config


Simple we can say n no's of VM's can create in HYPERV


The number of VMs you'll be able to support will depend on several factors – the capacity of the server hardware, the efficiency of the hypervisor, and the requirements of the guest operating systems. Server hardware can support up to four 12-core processors, 256GB of RAM, and four or more quad-port Gigabit Ethernet or dual 10G Ethernet adapters, along with enough high-speed storage for dozens of VMs per server.

Efficiency relates to the ability of the hypervisor to recover unused resources when a VM is not in use. CPU resources and memory can be allocated only when needed. Storage space can also be thin-provisioned, which means that even though a VM has an 80GB virtual drive, only the 10GB or so actually in use to store files will occupy space on the storage system, rather than the full 80GB.

Of course, guest operating systems will vary widely in their requirements for CPU power, memory and storage. For example a Linux server might only need half a CPU core, 512MB of RAM and an 8GB virtual disk, while running Windows 7 optimally will require at least one CPU core, 2GB of RAM (preferably four), and 20GB or more of virtual disk. Not all Windows guests will require lots of resources – Windows Web Server 2008 R2 can operate with minimal resources. In general though, Linux will require less in the way of resources than other OSes.

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