Logical disk

A logical disk is a virtual 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 or "virtual" because it does not actually exist as a single physical entity in its own right. Most modern operating systems provide some form of logical volume management which allows the creation and management of logical volumes.

Logical disks can be 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)
• Storage area network (SAN) : If the SAN is virtualized, a device it presents logical disks to the host operating systems
• Storage Subsystem : Usually providing a redundant array of independent disks (RAID) where logical disks (partitions) are presented to the SAN from the disk arrays themselves. The arrays actually contain the physical disks.

Motivation

When IBM first released the magnetic disk drive in the 1956 IBM 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. This means that more than one physical disk are put into a RAID together to produce a single logical disk.

In a modern home personal computer, disk drives can provide hundreds to thousands 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.

Many modern business information technology environments use a storage area network (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 used the name VDisk, for "virtual disk".^[1]

Today, the rationale for the Logical disk approach starts to be questioned ^[2] and solutions that offer more flexibility and better abstraction are increasingly needed.

See also

• Logical volume management (LVM)
• RAID
• Storage Virtualization

References