Virtual Disk Service Glossary

[Beginning with Windows 8 and Windows Server 2012, the Virtual Disk Service COM interface is superseded by the Windows Storage Management API.]

This section provides a glossary of technical terms used in the Virtual Disk Service (VDS) documentation.

automagic configuration

Hardware RAID provider that supplies a set of rules for choosing LUN logical block remapping based on simple attributes. Automagic providers can also dynamically alter the mapping for performance or fault management.

automagic hints

Information supplied to an automagic provider to guide the LUN logical block configuration. Hints include information as to the desired fault tolerance, physical atomicity, and intended I/O access pattern.

basic disk

A disk managed by the simplest possible software provider. The basic disk provider supports only volumes that directly map to partition configuration data structures.


Selecting for a set of mappings to physical resources.


The process of converting a basic disk to a dynamic disk.


A collection of the operating parameters that supply the mapping from a volume, or LUN, to physical resources.


A software program that contains the runtime intelligence for a hardware provider.


A physical disk or hardware RAID LUN. Disks are targets for runtime storage I/O load; Plug and Play (PNP) does not distinguish between JBOD and LUNs.

disk platter

A subset of a pack used for exporting or importing volumes from a pack.

disk pack

See pack.


A physical disk spindle within a hardware RAID subsystem. Drives are bound into LUNs by the subsystem.

dynamic disk

A disk managed by a software RAID provider with support for flexible volume reconfiguration. A dynamic disk uses a partition as a container for volumes; there is no guaranteed mapping.

explicit configuration

A configuration with the parameters, including the volume type and the exact layout, for a collection of target volumes.


The act of moving a disk platter and all volumes contained on that platter out of a pack.


A contiguous range of logical sectors contributing to a single volume or disk. An extent can also be range of unallocated sectors. Commonly, a file system displays the extent details to an end-user.

GUID Partition Table (GPT)

Structures used by EFI firmware. GPT is one of two lowest level software configuration data formats used by platform firmware to locate a bootable operating system or other executable image.

hardware provider

A collection of software that executes on the host, a bus adapter, and possibly an external storage subsystem that works together to surface and manage RAID LUNs. Hardware providers for most external storage subsystems contain only user-mode, host-based software.


The current fault-management status of a volume or a LUN.

hot sparing

Temporarily adding a volume plex to a volume.


The act of moving a disk platter and all its volumes into an existing pack.

just a bunch of disks (JBOD)

A collection of physical spindles without the coordinated control provided by a hardware RAID device.

logical block number (LBN)

The smallest addressable unit of storage data. LBN is used with I/O command protocols.

logical block mapping

The transformation of the logical blocks presented to a provider to those exposed by the provider.

logical unit number (LUN)

A physically addressable storage unit as surfaced by a hardware RAID subsystem. This term can also refer to the SCSI identifier for the storage unit.

LUN number

A number that a VDS hardware provider assigns to a LUN in an array. This is not the same as the "logical unit number" in the disk's SCSI address.

management service

A software program that executes as needed to perform volume configuration, monitoring, or fault handling.


A volume that is exposed to the operating system and has an associated volume name (a drive letter). A volume is accessible by a file system.


A disk not recognized by the operating system. A disk can be masked by the hardware RAID subsystem, switch, SCSI RESERVE by another computer host, or software in the disk stack.

master boot record partitioning (MBR)

MBR is one of two lowest level software configuration data formats, and is used by BIOS firmware to locate a bootable operating system image.


A collection of concatenated disk extents contained on one more disks. A disk can contribute to only one member of a volume.


A volume or disk mapping that maintains two or more identical data copies. The type of mapping is also called RAID Level 1.


A collection of logical volumes and underlying disks. A pack is the unit of transitive closure for a volume. Dynamic disk packs and disk groups are terms for the same item.

parity stripe

A volume or disk mapping that maintains parity check information as well as data. Each vendor supplies the exact mapping and protection scheme. Includes RAID 3, 4, 5, and 6.

partially directed configuration

A volume or disk configuration that is not fully explicit. The binding type and a collection of target resources are specified; the actual layout is not. Partially directed configuration is the most common volume configuration.


A contiguous range of logical sectors described by a single entry in the MBR or GPT structure. On basic disks, partitions directly correspond to simple volumes. Partition structures are shared between the BIOS or EFI platform firmware and an operating system or other bootable image.


The access path from a computer to a disk or external hardware LUN.


One member of a mirrored volume or LUN.


The endpoint of a path at a disk.

redundant array of independent disks (RAID)

A collection of techniques for managing multiple disks.

runtime service

A software program that executes on an I/O-request basis.

simple disk

A spindle that is not connected to a hardware RAID controller. See also Just a Bunch Of Disks (JBOD).

software provider

Host-based software that exposes logical volumes and operates on disks. A software provider includes kernel-mode runtime services, configuration data, and user-mode management services.


A volume or disk linear mapping of multiple discontinuous disk or drive extents. The extents can be on one or more spindles or LUNs.


A physical unit of disk storage.


The act of constructing a volume or LUN by performing more than one logical block mapping operation. An example is a mirrored striped volume. The most common stacking occurs when software RAID is used across LUNs constructed by hardware RAID.


A volume or disk mapping that interleaves contiguous extents across multiple volumes, disks, or drives. This mapping is also called RAID 0.


The current availability of a volume, disk, or drive.


The instantiation of hardware-provider software. A subsystem contains at least one controller and one drive. If the subsystem is external to the computer, one or more network paths connect the subsystem to the computer.

transition state

Status of the logical to physical mapping that is undergoing change.

uninitialized disk

A disk that is not a member of a pack.

unmasked disk

A disk that is visible to the operating system. The contents of an unmasked disk are visible to software volume managers, file systems, and so on.


A number of disk extents bound into a virtually contiguous range of logical blocks. A volume can be accessed through a drive letter, a mounted folder, or a volume GUID path.