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Glossary of RAID Terms
Terms and Expressions Commonly Used – Alphabetic

NOTE:  Definitions provided are applicable expressly to RAID data storage equipment technology comprised of hard disk drives (HDDs) and are narrowed to exclude variations of definition that may apply other meanings within the broader scope of computer and information technology generally.

array:   the referent used to collectively indicate the entire group of physical drives being combined to form a single data storage resource or subsystem;  will typically appear to the host system as a single drive;  pertains to the second letter of the "RAID" acronym.

availability:   the name for the metric whose value is directly proportional to redundancy and is an indicator for the level of fault tolerance provided by a given RAID configuration;  a state or condition of stored data where all recorded information is fully and easily accessible.

backup:   (noun) a duplicate or secondary copy of file data made for protection against sudden loss of primary file data; (verb) replicating available/accessible computer file data or sometimes recovering/recreating lost file data; ~ with zero protection gap is best, longer/larger gap reduces effectivity, when gap is excessive, ~ may be rendered useless.

CDP:   Continuous Data Protection; supercedes legacy data backup strategies; designed to eliminate protection gaps; can obviate the need for data recovery emergencies.

checksum:   an algorithmically generated numerical value used for error checking, detection and, if the error is within certain parameters or bounds, it can serve as the basis for correcting the error; a form of hash-code/function; related to CRC (cyclic redundancy check) codes; see parity.

controller:   the electronic device organizing the overall RAID structure;  on the "front end" connects to the host system appearing as a single physical drive, on the "back end" connects to each individual member hard disk drive in the array;  the particular design features of this component, not the disk drives, determines the operational nature and character of the operator's experience both for storing/retrieving data upon the subsystem and for system operation and maintenance by means of producing control and status displays for the operator.

degraded:   the state of RAID operation when redundancy has been reduced or eliminated as a result of an array member (typically an HDD) being taken off-line or disconnected from the array by the RAID controller logic;  a state of RAID operation where no loss of access to stored data has taken place, but the nominal level of fault-tolerance is no longer present;  takes place whenever an active HDD or flash-memory SSD array member fails; e.g., one drive fails and RAID-5 becomes equivalent to RAID-0, or RAID-6 becomes equivalent to RAID-5;  a state of impending RAID subsystem failure that unfortunately often goes unnoticed by users and is frequently ignored by system operators.

device:   a computer hardware unit or instrument invented, designed, and produced to fulfill a particular function facilitating a specific purpose within a subsystem or other scheme/strategy for storage of data.
disk:   a drive using rotating magnetic platters for recording information; see "drive" and "hard disk"; an element or member unit of an array; spelled "disc" by Seagate Technology.
downtime:   lost time or resource availability disruption caused by the cessation of computing system function when subsystem data are unavailable because the mass storage device(s) containing them are off-line or have failed; temporary, short duration loss may occur due to routine system maintenance or RAID array rebuild operations, while relatively long term disruption would follow when data without redundancy is lost, either temporarily or permanently.
drive:   a computer memory unit, either physical or logical, serving as a repository for digital information, storing new or additional data up to its maximum capacity, and making this data accessible, retrievable, or available at any later time on demand of the computer system; the physical memory unit may be implemented with any of various data storage devices, whether mechanical spinning disk technologies or all electronic solid-state devices (SSD); an entire RAID subsystem may be referred to as one of these.
EBOD:   Extended Bunch Of Disks; drives, HDDs, RAIDs, NAS, etc., used collectively or independently to implement or enhance scalability and flexibility within a storage network; logically not an interactive array of any special kind, but a specific, referential storage resource grouping — the host system will indicate at least one name/ID or "drive letter" per storage entity contained within the grouping.  [A newly coined term, circa 2012.]
efficiency
  (of storage capacity use):
  a value expressing the amount of available data storage capacity for stored information relative to the overall sum of storage capacity needed to implement a given level of RAID; formulaically expressed as "n+p", where "n" is the number of member drives needed to provide for the actual array capacity, and "p" is the number of member drives needed to provide additional capacity for redundancy, thus the larger the "n" value in proportion to the "p" value, the greater the efficiency.
element:   one of multiple identical data storage components, e.g., a physical HDD or flash-memory SSD within a grouping of like units, collectively constituting an array; usually synonymous with "member".
fault tolerance
(loss protection factor):
  the degree to which a subsystem can withstand the failure of member components while maintaining data availability; the name of the metric for rating the capability of a data storage subsystem to withstand storage element failure; the feature that provides for the fundamental, primary reason behind all RAID implementation (except RAID-0).
flash memory:   all electronic, non-volitile, non-mechanical, semiconductor data storage technology comprising the information retension media essential to SSD drives and appliances;  the data storage technolgy used in arrays with SSD members.
hard (disk) drive:   HDD; a physical as opposed to a logical drive; magnetic digital data memory device using rotating/spinning disk platters coated with a magnetic medium designed for recording (writing) and play-back (i.e., reading) of information, thus facilitating the storage of data.
host system:   the computer processing system directly connected to, or in direct communication with the RAID controller for a given RAID data storage subsystem.
hot spare:   any single HDD unit residing within a subsystem, and subject to automatic, ongoing confirmation of its operational status by means of the RAID controller, but not being used to store data stripes, and, which can be automatically brought on-line as an active array member by the RAID controller in the event of a fault in another array member HDD (which has been first taken off-line); an array member not active in storing any part of data currently stored on the subsystem array to which it belongs is not a "hot" spare unless integration as an active member is automatic.
JBOD:   Just a Bunch Of Disks; HDDs used collectively or independently and not configured as RAID; logically not an array of any special kind, only a physical grouping — the host system will show at least one "drive letter" per device contained within the grouping (if any given device is logically partitioned more drive letters will show up correspondingly).
layer:   an instance of fault-tolerance; implementation of data redundancy design capable of tolerating one drive member failure while keeping all data stored on the subsystem available to the host system; additional ~ will create increased data availability.
member:   typically an HDD, but possibly any physical or logical data storage device belonging to an array; usually synonymous with "element".
mirroring:   achieving redundancy by use of two hard disk drives (or other type of memory or data storage devices), typically identical, to hold the same identical data at all times; synonymous with RAID-1; using two identical subsystems, arrays, or groups of data storage devices to hold separate, identical, concurrent copies of data.
nesting:   when one or more members of a RAID consists of another RAID; characterized by an array acting as a single element within a higher order array.
off line:   the status assigned to a specific, individual array member / element / data storage device within a RAID whenever the controller detects any fault with that unit.
OS:   Operating System; for the majority of the world's computer users, Windows® is the best known example; often referred to as the system platform; the entity serving as host to the file system.
parity:   information generated algorithmically by a RAID controller within a set of corresponding stripes written upon each successive member of an array producing a kind of error-correction that permits the aggregate data contained in each stripe-set to be complete even when one (and only one) of the stripes within the set is missing, thus enabling fault-tolerance; information that, when combined with available data, provides the basis for regenerating missing data (error correction); related to, and used for the generation of a checksum.
performance:   most widely understood generally as a measure of data output/input (read/write) operations per specified period of time; more specifically to RAID there are at least two additional factors of lesser importance:  the efficiency of storage element use and, also worth considering, is the subsystem "behavior" or the user experience as affected by operational reporting from the subsystem hardware, controller and software.
persistent storage:   Allocated, active data storage space with a specific physical address.
physical address:   Addressable data storage location on a specific hardware storage device, which is a particular HDD or member of an array, within a specific sector or cluster upon that device's allocated, active data storage space.
protection gap:   generally, the length of time between backup operations; the shorter the length of time (the smaller the ~), the better the level of protection.
RAID:   Redundant Array of Inexpensive Disks (the earliest known interpretation, both the words "individual" and "independent" are often given nowadays for the  " I "  word); a data storage subsystem consisting typically of drives with data "striped" across the members of the array.  Please see "View of RAID Types" and "Level/Type Definitions" for detailed information.
RAISE:   Redundant Array of Independent Silicon Elements; also known as a flash-array, essentially it a RAID data storage subsystem consisting of SSD elements in place of spinning disk technology, but similarly having data "striped" across the members of the array.  [A newly coined term, circa 2012.]  Please see "View of RAID Types" and "Level/Type Definitions" for detailed information.
rebuild:   a procedural operation performed following replacement of a failed RAID member drive in order to reestablish fault-tolerance or a functional redundancy, restore a non-degraded state, and reinstate nominal data availability;  a maintenance operation where, for RAID-5, no more than one single hard drive (array element) may be replaced;  an inappropriate method for remediating a failed RAID subsystem (it never works) — see "The One Law".
redundancy:   the provision for multiple means of data access within an array;  in the event of array member drive fault, is the implementation of recording user information duplicates (via copies) or alternatively, means enabling the duplication of user information (via regeneration), thus protecting the RAID from data loss;  is increased to provide greater data availability within a given RAID subsystem; an increase of ~ generally decreases the storage efficiency;  pertains to the initial letter of the "RAID" acronym.
restore, restoration:   to bring inaccessible, unavailable, or otherwise off-line data back on-line; using objects of data protection to reinstate data availability and operation of stored data repository.
solid-state device:   SSD; a physical as opposed to a logical drive;  electronic, usually silicon based digital data memory device having no moving parts, typically using flash memory for recording (writing) and play-back (i.e., reading) of information, thus facilitating the storage of data;  increasingly, and gaining substantial adoption momentum by 2014, an enhancement to and replacement for mechanical, magnetic, spinning disk HDD technology as RAID array elements;  used in RAISE.
storage element:   a device used to store data; see "element", and "member".
stripe:   n.:  a short, small segment of data that has been recorded upon any single given drive element within a RAID subsystem.
stripe set:   the aggregate of one data stripe sequentially written in a corresponding position on each of consecutive active members and orchestrated by the RAID subsystem controller.
striping:   v.:  the process of a RAID controller dividing a body of data (typically one or more files) into small, short segments, typically 32Kb to 256Kb in length, and distributing these data segments, together with a checksum, and then writing them sequentially across each member of a RAID array — typically no complete data file exists on any one member (exception:  in certain cases where the file size is small enough, a single segment may contain sufficient space for a complete file).
striping cycle:   one iteration or rotation of writing a single stripe set of data onto each active member of a RAID subsystem.
subsystem:   a coordinated assemblage of data storage related devices working together to comprise a single data storage resource; an entity subordinate to a host system;  a pronoun for a RAID array or array-controller combination.
Super RAID:   a RAID design offering multiple layers of redundancy, or protecting against the concurrent failure of more than one data storage device or element; typically consisting of components, some or all of which are or could be stand-alone, single layer RAID configurations; see "Understanding RAID Levels".
uptime:   a metric for high data availability; often expressed as a percentage of overall running time (e.g., 99.999% = "five nines").
WAFL:   Write Anywhere File Layout; a file layout that supports large, high-performance RAID arrays, quick restarts without lengthy consistency checks in the event of a crash or power failure.
Z-end:   the last, ending, concluding, terminating, closing, final, only remaining, entry of this MicroCom Glossary of RAID Terms.

Compiled by:  S.E. Fowler / Steve Fowler



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