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Alogical disk,logical volume orvirtual disk (VD[1] orvdisk[2] for short) is avirtual device that provides an area of usable storage capacity on one or more physicaldisk drive(s) in a computer system. The disk is described aslogical orvirtual because it does not actually exist as a single physical entity in its own right. The goal of the logical disk is to provide computer software with what seems a contiguous storage area, sparing them the burden of dealing with the intricacies of storing files on multiple physical units. Most modernoperating systems provide some form oflogical volume management.
Logical disks can be defined at various levels in the storage infrastructure.
An operating system may definevolumes orlogical disks and assign each to one physical disk, more than one physical disk or part of the storage area of a physical disk. For example,Windows NT cancreate several partitions on ahard disk drive, each of which a separate volume with its ownfile system. Eachfloppy disk drive,optical disc drive orUSB flash drive in Windows NT becomes one volume. Windows NT can also createpartitions that span multiple hard disks drives. Each volume is identified with adrive letter.
Storage area networks (SANs) consolidate inhomogeneous storage devices. As suchlogical disks orvdisks allow computer programs to access files stored on a SAN.[1][2]
A hardware-levelredundant array of independent disks (RAID) exposes itself to the operating system as one logical disk while the array itself consists of several disks. The operating system either does not know that the hardware with which it is interfacing is a RAID, or knows but still does not concern itself with intricate details of storage. In case of the latter, specialized management, maintenance and diagnostics software dedicated to that specific RAID may run on the operating system.
WhenIBM first released themagnetic disk drive in the 1956IBM 305, a single disk drive would be directly attached to each system, managed as a single entity. As the development of drives continued, it became apparent that reliability was a problem and systems using RAID technology evolved, so that more than one physical disk is used to produce a single logical disk.
Many modern businessinformation technology environments use a SAN. Here, many storage devices are connected to many hostserver devices in a network. A single RAID array may provide some capacity to one server, and some capacity to another. Therefore, logical disks are used to partition the available capacity and provide the amount of storage needed by each host from a common pool of logical disks. TheIBM SAN Volume Controller uses the term "vdisk" to refer to these logical disks.[2]
Today, the rationale for the logical disk approach starts to be questioned[3] and solutions that offer more flexibility and better abstraction are increasingly needed.