Defining an Instance (Virtual Machine)
An instance (virtual machine) is a computing environment completed with virtual hardware devices which simulates the real computer system. The instance normally runs as a process in a physical computer system called host and you can boot an operating system inside the virtual machine. The instance will generally behave as if it were a real, physical machine.
Your instance's operating system is stored on a virtual hard drive it is called an image. You can launch a instance by using different images (operating system) and you can access it through SSH. As the instance thinks it runs in a 'real' computer system, you can do most things as with a normal computer system. However, there are still some cases that can be restricted by underlying virtual hardware support such as 3D graphics.
Instances add some overhead as there are virtualization layers to support required functions, so they won't be as fast as if you run the operating system on real hardware.
Why is virtualization useful
Virtualization is useful in the following scenarios:
Flexibility: If you need more instances or more computing power, simply launch more instances or launch a instance with more CPUs and RAMs.
Easier software installations: Instances can be used to ship the entire software configuration. The instances can be 'snapshot' into an image after configuration, and then other users can use the image to launch new virtual machines that are already set up.
Testing and disaster recovery: Once installed, a virtual machine can be consider as a "container" (snapshot) that can be arbitrarily frozen, woken up, copied, backed up, and transported between hosts.
Infrastructure consolidation: Virtual machines can significantly reduce hardware and electricity costs.
It is helpful to understand some crucial terminology, especially the following terms:
host: This is the computer system where your virtual machine is running on top of it.
guest operating system: This is the operating system that is running inside the virtual machine.
virtual machine image: a single file which contains a virtual disk that has bootable operation system installed on it.
This guide aims to provide more detailed information to advanced users who might act as system administrator to manage instances running in NeCTAR Cloud.