Virtualization and Disk Performance 1
Given that virtualization technologies have many specific applications this paper will begin by first presenting definitions. Definition: Virtualization Essentially to virtualize something means to make something that doesn’t actually (physically) exist appear to exist. Think of the context invirtual reality. Let’s make a quick example of something everyone in IT is familiar with, a PC with 4 logical volumes (C, D, E, and F). In reality that desktop has one physical disk drive partitioned into 4 volumes. A logical volume is in this case a virtual drive. Next we’ll define two popular modern applications of virtualization technology. Definition: Server Virtualization1 / Virtual MachineServer virtualization describes the creation of one or more virtual instances of a “guest” operating system either on top of a “host” operating system (Hosted Architecture) or directly on top of a specialized software layer called a hypervisor (Hypervisor Architecture). In either architecture, the host system’s virtualization of other operating systems is accomplished by software, proprietary to thevendor (e.g. Virtual Server™, VMware™, Virtuozzo™), which resides between the physical hardware (CPU, memory, etc) and the "guest" operating systems. Each guest or host operating system runs its own applications independently, as if it were the only system operating on the hardware.
Image1 (virtual machines)
In a host/guest environment, each instance of a guest operating system stores a filecalled a virtual disk (e.g. .vhd, .vmdk) on the host system. This is a very common implementation of machine virtualization today. Hypervisor architecture removes the requirement for a “host” system. With a hypervisor, virtual machines run on a thin layer of hardware abstraction software. That software layer, the hypervisor, addresses hardware communications for all the virtual systems on thatmachine. Hypervisor represents the future of virtual machine technology.
For the general purposes of this document, server virtualization also includes PC (workstation) virtualization.
Virtualization and Disk Performance 2
Definition: Storage Virtualization Storage virtualization involves the creation of a usually very large, logicalpool of data that, via software, appears to bephysically located all on one server. In actuality, that data may be located across hundreds of physical disks spread across dozens of servers. This is the concept implemented by Storage Area Networks (SAN). For peak performance these storage pools require automatic disk defragmentation just the same as a single hard drive would. Automatic defragmentation is implemented from server(s) that manage therespective logical disk volumes.
Image2 (storage virtualization)
Our last definition is a broad explanation of disk fragmentation. Definition: Disk Fragmentation Disk fragmentation, is the condition in which pieces of individual files and free space on a disk are not contiguous, but rather broken up and scattered around the disk. This requires the hard drive to locate all the fragments of afile. The collection of file fragments from numerous places instead of just one causes file access to take significantly longer than it should. File writes into fragmented free space, also take longer and can increase Image3 (disk fragmentation) the likelihood of newly created files fragmenting. The affect of disk fragmentation is slower system performance, increased I/O overhead, and more severecases, compromised reliability resulting in phenomena such as application and system hangs and crashes.
Depending on your perspective, virtualization’s purpose is to afford divergence and convergence. It affords the division of logical objects that should be separated, and/or the consolidation of objects that should be grouped together. The technology’s recent explosion coincides with...