When people speak about the benefits of virtualisation the main benefit they reference is consilidation of physical machines, but there are many other benefits to a virtual enviroment, and just a few drawbacks. This article discusses the Pro's and Con's of virtualisation, from a system manager's point of view.
When we talk about virtualisation we are talking about enterprise level , bare metal hypervisors that run directly on the hardware without the use of an intermediary operating system.
This article is not relavant to desktop virtualisation products like VMWare Workstation, Sun Virtual Box , VMWare Fusion on the Macintosh and so on. These products are really just meant to provide an easy way use more than 1 operating system on a desktop computer.
Enterprise virtualisation offers highly available 24x7 services. It has been at the front line of consolidation for the past few years and will continue to do so for sometime.
However as the title suggests the benefits do not stop there, in some aspect virtual machines ( or VM's ) are more manageable than regular physical machines. What the virtualisation vendor's do not shout about is the limitations of virtualisation of which there are not so many, but they are a more quietly kept secret.
Physical Server Consolidation
Whilst VMware ESX does squeeze more more virtual machines in less physical space, it also manages and redistributes resources. Because ESX is a bare metal hypervisor the over head is low, however the servers that you run ESX on will be higher spec with more redundant features. These machines do not come cheaply.
For a little extra money you might also seek the more effecient servers that produce less heat and draw less power, which may or may not fit into your needs depending on your power and cooling limitations, this is particulary a problem is decade + old data centers which were built for bigger machines taking more space and producing less heat.
One thing is for sure, one rack of 42u can pack enough ESX bare metal hardware and storage to house 1000's or 10000's of virtual machines that would have previously required its own data center.
CPU / Memory Consolidation
CPU and memory resources are shared by the pool of virtual machines, so less total CPU and memory is needed per machine.
Resources pools ( where configured ) will priortize the usage of resources. These resource pool rules are only activated when there is a contention for resource , under normal conditions where the VM host is not saturated the resources are granted as needed, but when there is more demand than resources available the rules come into play.
ESX essentially shares the free CPU cycles and free RAM memory with other machines, keeping track and managing the physical hosts resources making sure each VM has its allocated memory and CPU quota available, depending on the resource configuration it can supply more resources if the VM needs them and the resources are available within the ESX server or cluster to provide performance under high usage.
