What Is A Hypervisor?

Something you will likely hear a lot about when accessing information regarding Virtual Private Servers (VPS) is the term ‘hypervisor’.  So what is one and what does it do?
Firstly, let’s have a brief look at how server virtualisation works.  A Virtual Private Server is essentially a fully autonomous server that runs using a specified number of resources on a real hardware server, often called the host server.  This works by having software that defines the memory, disk and CPU allocations for the Virtual Private Server and manages its use of the hardware in the underlying machine.  This piece of software is referred to as a ‘hypervisor’.
The hypervisor works by looking at the resources allocated to a particular virtual machine, then making sure that that resource is then transferred through onto the hardware.  For example, the memory that the Virtual Private Server is allocated will be partitioned out of the memory installed inside the hardware host server.  This memory allocation will be kept separate from any memory assigned to other Virtual Private Servers and the host server’s operating system.  Similarly, the use of CPU is broken down into the number of CPU cores designated to the Virtual Private Server and the amount of time that is to be scheduled.  The hypervisor software then manages how the CPU time of the hardware host is given to the Virtual Private Server to meet its allocation.  The benefit here of virtualisation with a hypervisor over technologies like emulation is that the virtual server’s processes run directly on the hardware CPU, whereas with emulation there is an emulated CPU that provides abstraction from the underlying hardware, dramatically reducing performance.
Emulation is thus an emulated service running on a virtual CPU managed by the underlying operating system using the physical CPU, virtualisation directly accesses the physical CPU theoretically making the virtual server much more flexible and powerful.
Virtualisation through a hypervisor can lead to other performance benefits such as the potential to adjust the resource allocation on the fly with the hypervisor software increasing the amount of CPU time or cores that the Virtual Private Server can use, or increasing the size of the memory partition it can use.  This also includes the ability to decrease allocation accordingly as the situation requires.  Together this means that the Virtual Private Server can have its resources scaled easily to meet the demands placed on it.
Hypervisors tend not to function in terms of disk space as these are generally partitioned from the available disk using technologies such as Linux’s Logical Volume Management or by creating large files within an existing file system, referred to as disk images.  As such, these are usually managed by different software to the hypervisor itself, and often manipulation of the disk images can mean the hypervisor has to stop running the Virtual Private Server while the changes take place.
In terms of Cloud VPS infrastructure these two distinct sections of the Virtual Private Server get separated onto different dedicated hardware.  Storage Area Networks (SAN) handle the disk images and allocations of space while powerful servers with large amounts of memory run the hypervisor processes.  With this partitioning of work, the hardware servers performing the various tasks come to be referred to by the task they perform.  As such, the host hardware server that runs the hypervisor software and thus the Virtual Private Servers it manages come to be referred to simply as hypervisors.  Similarly, the storage servers forming the Storage Area Network come to be referred to as SANs.
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