A logical disk, logical volume or virtual disk (VD or vdisk for short) is a virtual device that provides an area of usable storage capacity on one or more physical disk drive(s) in a computer system. The disk is described as ''logical'' or ''virtual'' 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 modern operating systems provide some form of

logical volume management In computer storage, logical volume management or LVM provides a method of allocating space on mass-storage devices that is more flexible than conventional partitioning schemes to store volumes. In particular, a volume manager can concatenate, ...

Levels

Logical disks can be defined at various levels in the storage infrastructure.

Operating system

An operating system may define ''volumes'' or ''logical 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 Windows NT is a Proprietary software, proprietary Graphical user interface, graphical operating system produced by Microsoft as part of its Windows product line, the first version of which, Windows NT 3.1, was released on July 27, 1993. Original ...

can create several partitions on a

hard disk drive A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating hard disk drive platter, pla ...

, each of which a separate volume with its own file system. Each

floppy disk drive A floppy disk or floppy diskette (casually referred to as a floppy, a diskette, or a disk) is a type of disk storage composed of a thin and flexible disk of a magnetic storage medium in a square or nearly square plastic enclosure lined with a ...

optical disc drive In computing, an optical disc drive (ODD) is a disc drive that uses laser light or electromagnetic waves within or near the visible light spectrum as part of the process of reading or writing data to or from optical discs. Some drives can on ...

USB flash drive A flash drive (also thumb drive, memory stick, and pen drive/pendrive) is a data storage device that includes flash memory with an integrated USB interface. A typical USB drive is removable, rewritable, and smaller than an optical disc, and u ...

in Windows NT becomes one volume. Windows NT can also create partitions that span multiple hard disks drives. Each volume is identified with a drive letter.

Storage area network

Storage area network A storage area network (SAN) or storage network is a computer network which provides access to consolidated, block device, block-level data storage. SANs are primarily used to access Computer data storage, data storage devices, such as disk ...

s (SANs) consolidate inhomogeneous storage devices. As such ''logical disk''s or ''vdisk''s allow computer programs to access files stored on a SAN.

Storage subsystem

A hardware-level

redundant array of independent disks RAID (; redundant array of inexpensive disks or redundant array of independent disks) is a data storage virtualization technology that combines multiple physical data storage components into one or more logical units for the purposes of data redu ...

(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.

Motivation

When

IBM International Business Machines Corporation (using the trademark IBM), nicknamed Big Blue, is an American Multinational corporation, multinational technology company headquartered in Armonk, New York, and present in over 175 countries. It is ...

first released the

magnetic disk Magnetic storage or magnetic recording is the storage of data on a magnetized medium. Magnetic storage uses different patterns of magnetisation in a magnetizable material to store data and is a form of non-volatile memory. The information is ac ...

drive in the 1956

IBM 305 The IBM 305 RAMAC was the first commercial computer that used a moving-head hard disk drive (magnetic disk storage) for secondary storage. The system was publicly announced on September 14, 1956,

, 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 business

information technology Information technology (IT) is a set of related fields within information and communications technology (ICT), that encompass computer systems, software, programming languages, data processing, data and information processing, and storage. Inf ...

environments use a SAN. Here, many storage devices are connected to many host server 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. The

IBM SAN Volume Controller The IBM SAN Volume Controller (SVC) is a block storage virtualization appliance that belongs to the IBM System Storage product family. SVC implements an indirection, or "virtualization", layer in a Fibre Channel storage area network (SAN). Arch ...

uses the term "vdisk" to refer to these logical disks. Today, the rationale for the logical disk approach starts to be questioned and solutions that offer more flexibility and better abstraction are increasingly needed.

References