服务器虚拟化

Server Virtualization, like other virtualizations, is a software architecture that allows multiple server operating systems to share utilization of the same underlying hardware infrastructure. The key enabling software is called a hypervisor, which creates and manages resource sharing between virtual environments, called virtual machines (VMs), and underlying infrastructure.

The alternative to server virtualization is to maintain separate server operating systems on separate hardware, each with their own pool of dedicated resources. While this is potentially less complex to implement, resource dedication risks leaving significant hardware resources underutilized.

Imagine, three OSes on three equal machines, each utilizing only 30% capacity, leaving 70% underutilized across all three systems. What should happen if the needs of one machine grows beyond its capacity? Can it borrow from the space available on the other machines?

With server virtualization, the resources of all three machines can be pooled, each OS can be placed into its virtual machine (VM), and resources can be shared with those VMs that need it most. If one VM needs additional space, it borrows from the resource pool unaware which machine those resources actually reside.

The benefits of virtualization derive from this main point, that resources can be pooled and shared as needed, and every last compute cycle and empty disk drive can be maximized with little waste. In short, virtualization brings the following benefits.

• Maximization of infrastructure capabilities
• Optimization of system resources
• Reduction in resource costs
• Increase in ROI on system uptime and hardware components
• Improvement in system usage and efficiency, app and data availability
• Ensured system resiliency and disaster recovery
• Ensured data protection and system security
• Centralized management

Server virtualization benefits organizations in the same fashion other virtualizations benefit them:

• Reduce IT Expenses
• Enhance System Resiliency and Disaster Recovery and Reduce Downtime
• Increase Efficiency and Productivity
• Enhanced DevOps and Virtualized Sandboxing
• Reduced Environmental Footprint

Virtualization enables the abstraction of operating systems away from hardware resources and can be achieved in multiple ways.

Full Server Virtualization — Full server virtualization is the installation of a hypervisor solely responsible for creating and monitoring virtual machines and managing commands between hardware, host OS, and VMs. The main advantage is that no modifications to guest operating systems is necessary, but the trade off to emulated environments is more complex, and typically slower.

Paravirtualization — Paravirtualization is somewhat like a stripped down version of full server virtualization. In full server virtualization, hardware I/O is emulated, and the hypervisor traps commands from VMs before making calls on hardware. In paravirtualization, guest OSes are modified to interact directly with the hypervisor eliminating the emulation overhead in full server virtualization. Minus emulation, performance improves, however, guest operating systems require modification.

Hardware Assisted Virtualization — In hardware assisted virtualization, virtualization operations remain similar to full and para virtualization, however, guest OSes, by virtue of using common x86 instruction sets, can then interface with modified hardware with much less hypervisor overhead. Performance improves with less hypervisor overhead, but modifications to hardware are required.

Kernel Level Virtualization — In this scenario, virtualization instructions are included within a Linux kernel, acting as its own hypervisor, and subsequent VMs are created and managed as if they were processes interacting through specialized device drivers. While kernel level virtualization has less overhead it does also require hardware support.

OS Level or System Level Virtualization — OS level virtualization shares a single kernel layered on a host operating system with multiple VMs. There are many advantages to these configurations, particularly extreme lightweight, enhanced environment isolation and security, live migrations, capability to host many VMs, and more. The main limitation of system level virtualization is that all VMs must run the same OS. This differs from server virtualization which is capable of running multiple brands of operating systems. There is a significant disadvantage to this configuration, potential kernel or driver issues can collapse all VMs.