An Intro to Virtualization with VMware vSphere 4
BackgroundVMware officially released vSphere 4 on the 21st of April, with all the fanfare you would expect to accompany a major product launch, however much of the focus was on how it would enable enterprises to move to "cloud computing". Whilst this all sounds great it is of little relevance to the SMB IT Admin, whose "datacentre" might run from a couple of servers to a couple of racks of servers with a constrained budget to match it.
Since our focus is on IT for those "Small to Medium Businesses" I have been evaluating vSphere 4 from that angle - looking at the features which are most relevant for those considering whether it is right for their network. This is the first of a series of articles to help you with your evaluation, which will discuss the key features of vSphere 4, subsequent articles will cover the actual process of installing and configuring the various components so you can test it yourself.
Virtualization TerminologyWhen discussing virtualization there are several key terms which can often cause confusion, in particular that of "server" which can be interchangeable between physical and virtual systems. Thus to avoid any confusion it is important to define the various terms employed by VMware and in this article:
Datacenters, clusters and hosts - The "host" is the basic building block of vSphere and refers to a physical server running the ESX hypervisor, whilst a cluster consists of two or more hosts with their associated resource pools, virtual machines and datastores. The datacenter is the largest unit of management in vSphere and contains one or more clusters.
Virtual Machine - the VM is the virtual equivalent of a physical server and as such has all the resources defined that you would usually expect when specifying physical hardware - CPU, memory, hard disks and networking. The operating system is then installed on the virtual machine in the usual way, the hardware is completely emulated by the hypervisor so that to all intents and purposes there is no significant difference between a virtual and a physical machine.
Virtual Appliance - Many vendors now supply pre-built virtual machines ready to deploy as a "virtual appliance", which may provide an easy way to test a server based product or increasingly as an alternative to a dedicated hardware appliance. A good example of this is Clearswift, who previously only supplied their web and email filtering systems as a hardware device but now offer a virtual appliance as an alternative. With vSphere Vmware have also introduced the "vApp", which extends the idea of the virtual appliance to enterprise level by allowing for the creation of multi-VM vApps with linked service and resource policies.
Hypervisor - the hypervisor is the software which provides the sharing and translation layer between the server hardware and the virtual machines running on top of it. The latest generation of Intel and AMD CPUs incorporate virtualization extensions which enhance this functionality by allowing more efficient sharing of resources whilst improving the isolation of individual VMs, so that one VM cannot cause another to crash.
Datastore - ESX storage is divided into datastores which are formatted with its proprietary VMFS file system, one of the key features of which is its ability to handle concurrent access by multiple hosts. Datastores can be created from any type of storage visible to the hypervisor but to enable most of the advanced vSphere features shared storage which is accessible to all the hosts in a cluster is required, i.e. a Storage Area Network. Datastore space is then provisioned into virtual disks which are attached to virtual machines to provide the storage for the operating systems installed on them.
Resource Pool - Many of the advanced features of vSphere revolve around the concept of a cluster of virtual hosts whose key resources (CPU MHz and memory) are concentrated into a central pool. This pool can be subdivided into more resource pools which can then be used to automatically manage the allocation of resources to virtual machines and enable prioritization of key VMs over less important ones.
Virtualization - Fundamental ConceptsvSphere 4 is VMware's name for the suite of applications that make up the latest major release of their virtualization solution and replaces their previous "Virtual Infrastructure" product suite. It is built around their ESX 4.0 hypervisor, and vCenter Server which manages the ESX hosts to provide the advanced functionality. VMware's direct competitors are Citrix, with their "Essentials" suite based on the XENserver hypervisor, and Microsoft with their Hyper-V hypervisor and System Center Virtual Machine Manager. Such is the competitiveness of the virtualization market, and the obvious value of gaining market share, that all three vendors are now giving away their core hypervisors for free. Although migrating from one solution to another is not impossible it is a major undertaking, which underlines the importance of choosing the right vendor from the start.
The fundamental concepts of virtualization are the same irrespective of vendor and require a significant change of mindset for the IT Manager used to the concept of a "server" being a package of hardware and software together. Turning the software "server" into a "virtual machine" makes it hardware independent as it just requires a hypervisor providing the necessary resources, and turning the hardware "server" into a "virtual host" by installing the hypervisor on it allows it to run as many virtual machines as it has resources for. Separating the software from the hardware in this way opens up a whole new spectrum of possibilities and also brings features that were previously "enterprise only" well within the reach of the smaller IT infrastructure.