Server Consolidation:
In organizations with many servers, it allows all servers to have ``virtually`` the same hardware, and therefore, their management is more homogeneous and predictable.
Virtualization of a system, usually referred to as a server, consists of making a complete machine, with its operating system, applications, data, configuration, etc., actually run on a different physical machine, called “host”, so That the “guest”, runs on an intermediate layer.
This means that a virtual machine is for all effects, a totally independent machine, and consumes the resources that are assigned to it from the host machine.
Advantages:
Reversal procedure before changes
It allows the machine to be produced, take a snapshot or photo of how it is, so that an installation of a patch, if it does not work correctly, allows us to back up quickly and with Minimum standstill times.
Flexible resource allocation
A common configuration is to place multiple virtual machines on a single high-power host, and as each machine's workloads vary, more processors and / or memory can be assigned to each virtual machine. This same operation on physical machines, in the first place, would have to be all of the same make / model, and it would mean longer stops, and the consequent cost in hours.
Maintenance of obsolete applications
Sometimes it happens that a certain application only works with an old operating system, or support is no longer available. These operating systems often do not work properly on modern machines, and relying on an old server without support or guarantees, is a very high risk. There are migration tools that allow to pass a physical machine to virtual, and then we can execute that virtual machine in modern servers with support in force.
Contingency plans
Allows to set up a very simple contingency plan with recovery times of less than one hour, since moving a virtual machine is completely transparent and independent of the hardware.
High availability solutions
In the clustering modes, it allows running virtual machines in cluster mode. In these cases, the fall of a host machine is recovered immediately, since the external requests would be served by another member of the cluster.
Energy saving and environment
it allows to execute several servers in a single physical machine, with the consequent energetic saving.
Disadvantages:
Performance
We could say that the performance of a virtual machine is about 10% lower, so that for CPU intensive applications and disk, may have a lower performance.
Cost
One thing to keep in mind is the cost of virtualization software, since you have to have: host machine licenses, virtualization platform licenses (some include the host license), and one operating system license per virtual machine (The latter requirement would be identical).
In a virtual machine, in addition, you can not use the economical Microsoft OEM licenses, because by definition, an OEM license is embedded with a certain hardware. As a virtual machine is actually software, it is not possible to acquire this type of license. This is an element to take into account when developing a virtualization project.