Examples range from the ability to multiplex many VMs on the same hardware to advanced virtualization features such as live migration and enhanced security.Īt the most basic level, decoupling enables time- and space-multiplexing of I/O devices, allowing multiple logical devices to be implemented by a smaller number of physical devices. Many of the benefits of virtualized systems depend on the decoupling of a VM's logical I/O devices from its physical implementation. After highlighting key benefits and challenges, we explore various implementation approaches and techniques that have been leveraged to enable flexible, high-performance I/O virtualization. Since virtualization is a broad topic, and the universe of I/O devices is large and diverse, this article focuses on some representative I/O systems issues in VM-based systems, primarily in the context of a single physical host. Today, VMs are commonplace in many computing environments and nearly ubiquitous in enterprise data centers and cloud-computing infrastructures. Since then, virtualization has experienced a resurgence of interest in both industry and academia. The term guest is commonly used to distinguish the layer of software running within a VM a guest operating system manages applications and virtual hardware, while a hypervisor manages VMs and physical host hardware.Īlthough IBM invented and commercialized mainframe VMs many decades ago, VMs didn't make the leap to commodity hardware until the late 1990s, when VMware pioneered efficient virtualization on x86 platforms. A virtualization software layer, known as a hypervisor, provides the level of indirection that decouples an operating system and its applications from physical hardware. Modern virtualization platforms exploit indirection and abstraction in numerous ways.Ī virtual machine (VM) is a software abstraction that behaves as a complete hardware computer, including virtualized CPUs, RAM, and I/O devices. Such indirection has proven to be remarkably powerful and versatile. The common theme is decoupling the logical from the physical, introducing a level of indirection between the abstract and the concrete. Other examples of virtualization include virtual LAN (VLAN), N_Port ID virtualization (NPIV), Intel Virtualization Technology for Directed I/O (VT-d), and Multiroot I/O Virtualization (MR-IOV). In computer architecture, an IOMMU (I/O memory-management unit) translates I/O-virtual memory addresses to corresponding physical memory addresses, making direct memory access by devices safe and efficient. In computer networking, a virtual private network (VPN) represents a logically isolated private network, where the isolation is provided using cryptographic methods to secure data that may in fact traverse the public Internet. Even within the narrower context of computer I/O, virtualization has a long, diverse history, exemplified by logical devices that are deliberately separate from their physical instantiations.įor example, in computer storage, a logical unit number (LUN) represents a logical disk that may be backed by anything from a partition on a local physical drive to a multidisk RAID volume exported by a networked storage array. ![]() ![]() The term virtual is heavily overloaded, evoking everything from virtual machines running in the cloud to avatars running across virtual worlds. Decoupling a logical device from its physical implementation offers many compelling advantages.
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