[Data Security] An unauthorized user who attempts to gain and/or succeeds in gaining access to an information system.


[Computer System] To stop all activity in a computer system in an orderly manner.

hard link

[File System] A path that provides a different name for a file.

Hard links are independent references to the same file; the file content is not deleted until every hard link to the file is deleted.

hard zone

[Fibre Channel] A zone consisting of zone members that are permitted to communicate with one another via the fabric.

Hard zones are enforced by fabric switches that prohibit communication among members not in the same zone on a frame by frame basis, based on the source and destination addressing. Well-known addresses are implicitly included in every zone.

hash value

[Data Management] A value deterministically derived from data and assumed to be unique enough within the domain of that data for the purposes of its application.

hash-based data deduplication

[Storage System] A method of performing data deduplication by calculating and comparing hash values.

See delta-based data deduplication.

Hashed Message Authentication Code (HMAC)

[Data Security] A value calculated over the contents of a message (usually using a cryptographic hash algorithm) that can be used to demonstrate that the contents of the message have not been changed during transmission.


[Computer System] Acronym for Host Bus Adapter.


[General] An approximation for a calculation that is too expensive to perform in its entirety.

Hierarchical Storage Management (HSM)

[Data Management] The automated migration of data objects among storage devices, usually based on inactivity.

Hierarchical storage management is based on the concept of a cost-performance storage hierarchy. By accepting lower access performance (higher access times), one can store objects less expensively. By automatically moving less frequently accessed objects to lower levels in the hierarchy, higher cost storage is freed for more active objects, and a better overall cost to performance ratio is achieved.

High Availability (HA)

[Computer System] The ability of a system to perform its function continuously (without interruption) for a significantly longer period of time than the reliabilities of its individual components would suggest.

High availability is most often achieved through failure tolerance. High availability is not an easily quantifiable term. Both the bounds of a system that is called highly available and the degree to which its availability is extraordinary must be clearly understood on a case-by-case basis.

High Performance Parallel Interface (HIPPI)

[Network] [Standards] An ANSI standard for an 800 Mbit/second I/O interface primarily used in supercomputer networks.

The subsequent 6400 Mbit per second I/O interface standard, HIPPI-6400, is more commonly referred to as the Gigabyte System Network (GSN) standard.

high speed serial direct connect

[Fibre Channel] A form factor that allows quick connect/disconnect for Fibre Channel copper interfaces.


[Network] [Standards] Acronym for High Performance Parallel Interface.


[Data Security] Acronym for Hashed Message Authentication Code.


[Computer System] A host computer.

host adapter

[Computer System] Synonym for host bus adapter.

host based array
host based disk array

[Storage System] A disk array whose control software executes in one or more host computers rather than in a disk controller.

The member disks of a host-based array may be part of different disk subsystems. See controller based array.

host based virtualization

[Computer System] Virtualization implemented in a host computer.

host bus

[Computer System] Synonym for host I/O interconnect.

Host Bus Adapter (HBA)

[Computer System] An I/O adapter that connects a host computer bus to an I/O interconnect.

Adapter is the preferred term for Fibre Channel and SCSI interconnects. The term NIC is used for networking interconnects such as Ethernet and token ring.

host cache

[Storage System] A cache that resides within a host computer whose primary purpose is to improve disk or array I/O performance.

Host cache may be associated with a file system or database, in which case, the data items stored in the cache are file or database entities. Alternatively, host cache may be associated with the device driver stack, in which case the cached data items are sequences of disk blocks. See cache, controller cache, disk cache.

host computer

[Computer System] Any computer system to which disks, disk subsystems, or file servers are attached and accessible for data storage and I/O.

Mainframes, servers, workstations and personal computers, as well as multiprocessors and clustered computer complexes, are all referred to as host computers in SNIA publications.

host environment

[Computer System] A storage subsystem’s host computer or host computers, inclusive of operating system and other required software instance(s).

The term host environment is used in preference to host computer to emphasize that multiple host computers are being discussed, or to emphasize the importance of the operating system or other software in the discussion.

host I/O interconnect

[Computer System] An I/O interconnect used to connect a host computer’s host bus adapter to storage subsystems or storage devices.

See I/O interconnect, channel.

host-side data deduplication

[Capacity Optimization] Deprecated synonym for source data deduplication.

hot aisle/cold aisle

[Energy] Arranging IT equipment in racks such that heat is exhausted in designated aisles while cool air is supplied in the alternating aisles.

hot backup

[Data Recovery] Synonym for online backup.

See cold backup, offline backup.

hot banding

[Storage] Inserting ranges of addresses that are accessed with greater frequency into a synthetically generated workload.

Hot banding is intended to reward caching behavior on the part of the storage system being measured.

hot disk

[Storage System] A disk whose capacity to execute I/O requests is saturated by the aggregate I/O load directed to it from one or more applications.

hot file

[File System] A frequently accessed file.

Hot files are generally the root cause of hot disks, although this is not always the case. A hot disk can also be caused by operating environment I/O, such as paging or swapping.

hot spare (disk)

[Storage System] A disk being used as a hot standby component.

hot standby (component, controller)

[Computer System] A redundant component in a failure tolerant subsystem that is powered and ready to operate, but that does not operate as long as all of its target primary components are functioning.

Hot standby components increase storage subsystem availability by allowing systems to continue to function when a component such as a controller fails. When the term hot standby is used to denote a disk, it specifically means a disk that is spinning and ready to be written to, for example, as the target of a rebuilding operation.

hot swap

[Computer System] The substitution of a replacement unit (RU) in a system for a defective unit, where the substitution can be performed while the system is performing its normal functioning normally.

Hot swaps are physical operations typically performed by humans. See automatic swap, cold swap, warm swap.

hot swap adapter

[Computer System] An adapter that can be hot swapped into or out of an intelligent device.

Some adapters that are called hot swap adapters should more properly be termed warm swap adapters, because the function they perform is interrupted while the substitution occurs.


[General] The term used to describe flip/flopping; when an opinion continually switches back and forth between two or more choices.

For example: Design choice "A" is selected; but a week later, design choice "B" is selected; then after another week of consideration, the design choice is switched back to "A".


[Data Recovery] Acronym for Hierarchical Storage Management.


[Fibre Channel] Acronym for High Speed Serial Direct Connect.


[Standards] Acronym for HyperText Markup Language.


[Standards] Acronym for HyperText Transfer Protocol.


[Network] A communications infrastructure element to which nodes on a multi-point bus or loop are physically connected.

Commonly used in Ethernet and Fibre Channel networks to improve the manageability of connecting devices to a bus structure, both managing physical cables and supporting the addition or removal of nodes from the bus while it is operating. Hubs maintain the logical loop topology of the network of which they are a part, while creating a “hub and spoke” physical star layout. Unlike switches, hubs do not aggregate bandwidth.

hub port

[Fibre Channel] A port on a Fibre Channel hub whose function is to pass data transmitted on the physical loop to the next port on the hub.

Hub ports include loop healing port bypass functions. Some hubs have additional management functionality. There is no definition of a hub port in any Fibre Channel standard.

hybrid array

[Storage System] A storage array consisting of multiple types of storage devices.

hybrid cloud

[Cloud] A composition of two or more clouds (private, community, or public) that remain unique entities but are bound together by standardized or proprietary technology that enables data and application portability.

Hybrid DIMM

[Hardware] A dual in-line memory module that contains multiple types of volatile and non-volatile memory technologies.

hybrid drive

[Storage System] A disk drive that consists of multiple types of storage media.

hyper-converged storage system

[Storage System] A storage product that combines server, client, storage, network, and management software oriented towards storage in a single unit.

HyperText Markup Language (HTML)

[Standards] A computer language consisting of a set of tags or “markup” codes that describe how a document is displayed by a web browser.

HTML tags are delimited by the characters "<" and ">". For example, the markup code "<p>" indicates that a new paragraph is beginning, while "</p>" indicates that the current paragraph is ending.

HyperText Transfer Protocol (HTTP)

[Standards] An application level protocol, usually run over TCP/IP, that enables the exchange of files via the World Wide Web.