Logical disk
Encyclopedia
A logical disk is a device that provides an area of usable storage capacity on one or more physical disk drive components in a computer system. Other terms that are used to mean the same thing are partition, logical volume, and in some cases a virtual disk (vdisk
).
The disk is described as logical because it does not actually exist as a single physical entity in its own right. There are many ways to define a logical disk or volume. Most modern operating systems provide some form of logical volume management
which allows the creation and management of logical volumes.
Logical disks are also defined at various levels in the storage infrastructure stack. From top to bottom :
first released the magnetic disk drive in the 1956 (IBM 305
) a single drive would be directly attached to the using system, with each disk 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. This means that more than one physical disks are RAID-ed together to produce a single logical disk.
In a modern home PC
environment, disk drives now provide hundreds of gigabytes of storage capacity which can be impractical to use as a single entity. Therefore, most systems have their drives partitioned into multiple logical drives.
In most modern business IT
environments, some form of a Storage area network
will exist. 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.
Vdisk
In storage virtualization, virtual disk, or vdisk, is a virtual logical disk or volume to which a host or application performs I/O operations. Generally provided by a storage virtualization device and applicable mainly to Fibre Channel storage area network infrastructures...
).
The disk is described as logical because it does not actually exist as a single physical entity in its own right. There are many ways to define a logical disk or volume. Most modern operating systems provide some form of logical volume management
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...
which allows the creation and management of logical volumes.
Logical disks are also defined at various levels in the storage infrastructure stack. From top to bottom :
- Operating System : Defines partitions on the disks to which it has visibility (these disks may be logical themselves)
- SAN : If the SAN is virtualized, a device in the same presents logical disks (vdiskVdiskIn storage virtualization, virtual disk, or vdisk, is a virtual logical disk or volume to which a host or application performs I/O operations. Generally provided by a storage virtualization device and applicable mainly to Fibre Channel storage area network infrastructures...
) to the host operating systems - Storage Subsystem : Usually providing some form of RAIDRAIDRAID is a storage technology that combines multiple disk drive components into a logical unit...
where logical disks (partitions) are presented to the SAN from the RAID arrays themselves. The RAID arrays actually contain the physical disks.
Why do we need them?
When IBMIBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...
first released the magnetic disk drive in the 1956 (IBM 305
IBM 305
The IBM 305 RAMAC, publicly announced on September 13, 1956, was the first commercial computer that used a moving head hard disk drive for secondary storage. RAMAC stood for "Random Access Method of Accounting and Control". Its design was motivated by the need for real-time accounting in business...
) a single drive would be directly attached to the using system, with each disk managed as a single entity. As the development of drives continued, it became apparent that reliability was a problem and systems using RAID
RAID
RAID is a storage technology that combines multiple disk drive components into a logical unit...
technology evolved. This means that more than one physical disks are RAID-ed together to produce a single logical disk.
In a modern home PC
Personal computer
A personal computer is any general-purpose computer whose size, capabilities, and original sales price make it useful for individuals, and which is intended to be operated directly by an end-user with no intervening computer operator...
environment, disk drives now provide hundreds of gigabytes of storage capacity which can be impractical to use as a single entity. Therefore, most systems have their drives partitioned into multiple logical drives.
In most modern business IT
Information technology
Information technology is the acquisition, processing, storage and dissemination of vocal, pictorial, textual and numerical information by a microelectronics-based combination of computing and telecommunications...
environments, some form of a Storage area network
Storage area network
A storage area network is a dedicated network that provides access to consolidated, block level data storage. SANs are primarily used to make storage devices, such as disk arrays, tape libraries, and optical jukeboxes, accessible to servers so that the devices appear like locally attached devices...
will exist. Here, many storage devices are connected to many host server
Server (computing)
In the context of client-server architecture, a server is a computer program running to serve the requests of other programs, the "clients". Thus, the "server" performs some computational task on behalf of "clients"...
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.
See also
- Logical volume managementLogical volume managementIn 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...
(LVM) - RAIDRAIDRAID is a storage technology that combines multiple disk drive components into a logical unit...
- Storage VirtualizationStorage VirtualizationStorage virtualization or storage virtualisation is a concept and term used within computer science. Specifically, storage systems may use virtualization concepts as a tool to enable better functionality and more advanced features within the storage system.Broadly speaking, a 'storage system' is...