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 :
- 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 (vdisk) to the host operating systems
- Storage Subsystem : Usually providing some form of RAID 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 IBM 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.
pl:Partycja logiczna simple:Logical disk