A logical disk, logical volume or virtual disk (VD or vdisk for short) is a

virtual device A virtual device in Unix is a file such as :/dev/null or :/dev/urandom, that is treated as a device, as far as user level software is concerned, but is generated by the kernel without reference to hardware. For instance when /dev/null is written ...

that provides an area of usable storage capacity on one or more physical

disk drive Disk storage (also sometimes called drive storage) is a general category of storage mechanisms where data is recorded by various electronic, magnetic, optical, or mechanical changes to a surface layer of one or more rotating disks. A disk drive is ...

(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 An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. Time-sharing operating systems schedule tasks for efficient use of the system and may also inc ...

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 graphical operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. Time-sharing operating systems sc ...

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 platters coated with magnet ...

, each of which a separate volume with its own

file system In computing, file system or filesystem (often abbreviated to fs) is a method and data structure that the operating system uses to control how data is stored and retrieved. Without a file system, data placed in a storage medium would be one larg ...

. Each

floppy disk drive A floppy disk or floppy diskette (casually referred to as a floppy, or a diskette) is an obsolescent 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 w ...

optical disc drive In computing, an optical disc drive 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 only r ...

USB flash drive A USB flash drive (also called a thumb drive) is a data storage device that includes flash memory with an integrated USB interface. It is typically removable, rewritable and much smaller than an optical disc. Most weigh less than . Since firs ...

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 In computer data storage, drive letter assignment is the process of assigning alphabetical identifiers to volumes. Unlike the concept of UNIX mount points, where volumes are named and located arbitrarily in a single hierarchical namespace, driv ...

Storage area network

Storage area network A storage area network (SAN) or storage network is a computer network which provides access to consolidated, block-level data storage. SANs are primarily used to access data storage devices, such as disk arrays and tape libraries from serve ...

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, RAID or Raids may refer to: Attack * Raid (military), a sudden attack behind the enemy's lines without the intention of holding ground * Corporate raid, a type of hostile takeover in business * Panty raid, a prankish raid by male college ...

(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 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 the use of computers to create, process, store, retrieve, and exchange all kinds of data . and information. IT forms part of information and communications technology (ICT). An information technology system (I ...

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

server Server may refer to: Computing *Server (computing), a computer program or a device that provides functionality for other programs or devices, called clients Role * Waiting staff, those who work at a restaurant or a bar attending customers and su ...

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). Arc ...

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