What is Virtualisation?

Virtualisation allows multiple virtual machines, with different operating systems to run in isolation, side-by-side on the same physical machine without interference. Each virtual machine has its own set of virtual hardware (for example, RAM, CPU, NIC, etc.) upon which an operating system and applications are loaded. The operating system sees a consistent, normalised set of hardware regardless of the actual physical hardware components.

How does Virtualisation work?

by Andy Archer.

Virtualisation is a technique whereby an operating system is provided with its I/O requirements by a special BIOS, in turn this BIOS is able to co-exists with many other BIOSes accessing the same hardware at the same time. These BIOSes are hosted by a ‘Hypervisor’ which co-ordinates the I/O from all the BIOSes as efficiently as possible onto the real hardware. The disk system for a Virtual Machine becomes a file under the control of the Hypervisor.

This special BIOS IS the Virtual Machine while the Operating System and application are guests within the Virtual Machine. The Virtual Machine is hosted alongside other Virtual Machines by the Hypervisor which contains the drivers for the real hardware. The Virtual machines all have the same virtual hardware albeit in different configurations, but clearly distinct from the real hardware.

The main areas of Virtual Machine I/O are

• Networking - virtual Ethernet to real Ethernet
• Storage -  virtual SCSI to real SCSI (using normal SCSI controllers, array controllers or fiber channel controllers)
• Memory – virtual x86 memory to real x86 memory
• CPU – to facilitate efficient virtualisation the actual CPU details are made known to the BIOS.

On its own this BIOS/Hypervisor technique provides an excellent platform for consolidation - that is many services delivered from one hardware platform, but when we introduce Hypervisor to Hypervisor communication another major benefit becomes apparent – Live Migration.

While a Virtual Machine is powered on, it has RAM, processor, disk and network access. Live Migration is the relocation of the Virtual Machine memory and processor settings from one Hypervisor to another over a network, an iterative update process ensures that the new location is able to assume control of the Virtual Machine with no effect on the service delivery.

The VMware VI3 suite implements virtualisation where the VMkernel component of ESX Server is the Hypervisor, and VMotion is the Live Migration technology. VMware VirtualCenter is the management application coordinating VMkernel to VMkernel activities hence providing VMotion and other distributed technologies such as HA for High Availability, and DRS for the dynamic re-balancing of Virtual Machine load distribution.

Distributed Power Management (DPM) is a new VI3 feature where, at times of low demand, Virtual Machines are consolidated onto a reduced number of physical servers and the un-used physical machines are suspended. When demand increases, Wake-on-LAN is used to bring the physical servers back into the cluster and a DRS / VMotion combination is used to re-balance the cluster.