[Data Security] Acronym for Remote Authentication Dial In User Service.


[Storage System] Acronym for Redundant Array of Independent Disks.

The phrase Redundant Array of Independent Disks is adapted from the 1988 SIGMOD paper A Case for Redundant Arrays of Inexpensive Disks. In modern systems, RAID techniques are often applied to storage devices or technologies other than disk.

RAID Level 0

[Storage System] Synonym for data striping.

RAID Level 1

[Storage System] Synonym for mirroring.

RAID Level 2

[Storage System] A form of RAID in which an error detecting code computed on stripes of data on some of a RAID array's disks is stored on the remaining disks and serves as check data.

RAID Level 3

[Storage System] A form of parity RAID in which all disks are assumed to be rotationally synchronized, and in which the data stripe size is no larger than the exported block size.

RAID Level 4

[Storage System] A form of parity RAID in which the disks operate independently, the data stripe size is no smaller than the exported block size, and all parity check data is stored on one disk.

RAID Level 5

[Storage System] A form of parity RAID in which the disks operate independently, the data stripe size is no smaller than the exported block size, and parity check data is distributed across the RAID array's disks.

RAID Level 6

[Storage System] Any form of RAID that can continue to execute read and write requests to all of a RAID array's virtual disks in the presence of any two concurrent disk failures.

Several methods, including dual check data computations (parity and Reed Solomon), orthogonal dual parity check data and diagonal parity have been used to implement RAID Level 6.


[Storage System] RAID 10 uses both striping and mirroring.

RAID 10 may be implemented in two different ways with different properties. One stripes sets of mirrored disk drives, which is often called “RAID 1+0”. The other mirrors sets of striped drives, which is often called “RAID 0+1”.

RAID array

[Storage System] Shorthand for Redundant Array of Independent Disks.


[Storage System] A form of RAID implemented in the open source ZFS project.


[Storage] Acronym for Redundant Array of Independent Nodes.

raised floor

[General] An elevated floor providing space for cable runs between equipment cabinets and cold air flow for cooling.

Many mainframe systems are designed to intake cool air from the bottom and exhaust heat from the top of a closed cabinet system.


[Storage System] A quantity of host system random access memory (RAM) managed by software and presented to applications as a high-performance disk.

RAMdisks generally emulate disk I/O functional characteristics, but unless augmented by special hardware to make their contents non-volatile, they cannot tolerate loss of power without losing data. See solid state disk.

random I/O
random I/O load
random reads
random writes

[Storage System] Any I/O load whose consecutively issued read and/or write requests do not specify adjacently addressed data.

The term random I/O is commonly used to denote any I/O load that is not sequential, whether or not the distribution of data locations is indeed random. Random I/O is characteristic of I/O request-intensive applications. See sequential I/O.

random number

[General] A number having properties of randomness or unpredictability.

There are three basic classes of random number. Deterministic or pseudorandom numbers are generated by an algorithm that produces a predictable sequence of values from an initial value called a seed. Cryptographically secure random numbers are produced in a sequence that, while deterministic, cannot be feasibly discovered or computed by examination of previous numbers in the sequence. Nondeterministic generators incorporate input from some unpredictable physical source that is outside human control.

random relative offset

[Fibre Channel] A transmission control algorithm in which the frames containing the subblocks that comprise a block of information may be transmitted in any order.


1. [Storage System] A set of physical disk positions in an enclosure, usually denoting the disks that are or can be members of a single array.

2. [Storage System] The set of corresponding target identifiers on all of a controller’s device I/O interconnects.

3. [Storage System] Synonym for a stripe in a redundancy group.

Because of the diversity of meanings attached to this term by disk subsystem developers, SNIA publications make minimal use of it.

rapid elasticity

[Computer System] Quick scaling of resources and capabilities to meet expansion and contraction of demand.

To the consumer, the capabilities available for provisioning often appear to be unlimited and available for purchase in any quantity at any time.

rapid provisioning

[Computer System] Quickly and automatically deploying services in response to requests.


1. [Computer System] Acronym for Reliability, Availability, and Serviceability.

2. [Windows] Acronym for Remote Access Server (Windows NT dialup networking server).

raw capacity

[Storage System] The sum total amount of addressable capacity of the storage devices in a storage system.

The addressable capacity of a storage device is commonly understood to be the number of bytes available to be written via SCSI or equivalent protocol. It does not include unaddressable space, ECC (error correcting code) data, remap areas, inter-sector gaps and so on. See theoretical capacity.

raw partition

[Storage System] A disk partition not managed by a volume manager.

The term raw partition is frequently encountered when discussing database systems because some database system vendors recommend volumes or files for underlying database storage, while others recommend direct storage on raw partitions.

raw partition backup

[Data Recovery] A bit-by-bit copy of a partition image.

A raw partition backup incorporates no information about the objects contained on the partition, and hence cannot be used for individual object restoration. See disk image backup.


[Data Security] Acronym for Role-Based Access Control.

read/write head

[Storage System] The magnetic or optical recording device in a disk.

Read/write heads are used both to write data by altering the recording media’s state, and to read data by sensing the alterations. Disks typically have read/write heads, unlike tapes, in which reading and writing are often done using separate heads.

ready idle

[Storage System] Synonym for idle.

real time copy

[Storage System] Deprecated synonym for mirroring.

real time data deduplication

[Storage System] Synonym for inline data deduplication.


[Storage System] The regeneration and writing onto one or more replacement disks of all of the user data and check data from a failed disk in a mirrored or RAID array.

In most arrays, a rebuild can occur while applications are accessing data on the array’s virtual disks.


1. [General] An interconnect or network device that includes a detector and signal processing electronics.

2. [General] A circuit that converts an optical or electrical media signal to a (possibly retimed) electrical serial logic signal.

3. [Fibre Channel] The portion of a Link_Control_Facility dedicated to receiving an encoded bit stream, converting the stream into transmission characters, and decoding the characters using the rules specified by FC-0.


[Network] The stationary (female) half of the interface connector on a transmitter or receiver.


[Storage System] Synonym for rebuilding.

Recorded Volume Serial Number (RVSN)

[Data Recovery] Synonym for media ID.


[Data Recovery] The recreation of a past operational state of an entire application or computing environment.

Recovery is required after an application or computing environment has been destroyed or otherwise rendered unusable. It may include restoration of application data, if that data had been destroyed as well.

Recovery Point Objective (RPO)

[Data Recovery] The maximum acceptable time period prior to a failure or disaster during which changes to data may be lost as a consequence of recovery.

Data changes preceding the failure or disaster by at least this time period are preserved by recovery. Zero is a valid value and is equivalent to a "zero data loss" requirement.

Recovery Time Objective (RTO)

[Data Recovery] The maximum acceptable time period required to bring one or more applications and associated data back from an outage to a correct operational state.


[Management] A DMTF open standard specification and schema that specifies a RESTful interface that utilizes JSON and OData for managing scalable hardware platforms.

Redfish schema

[Management] The representation of Redfish resources and data model using CSDL.

Redfish service

[Management] An OData service that conforms to requirements of the Redfish specification.

Redfish service entry point

[Management] A URI through which a particular instance of a Redfish Service is accessed.

reduced mode

[Storage System] Synonym for degraded mode.


[Storage System] The removal of a member disk from a RAID array, placing the array in degraded mode.

Reduction most often occurs because of member disk failure, however, some RAID implementations allow reduction for system management purposes.


[General] The inclusion of extra components of a given type in a system (beyond those required by the system to carry out its function) for the purpose of enabling continued operation in the event of a component failure.

redundancy group

[Storage System] A collection of extents organized for the purpose of providing data protection.

Within a redundancy group, a single type of data protection is employed. All of the usable storage capacity in a redundancy group is protected by check data stored within the group, and no usable storage external to a redundancy group is protected by check data within it.

redundancy group stripe

[Storage System] A set of sequences (strips) of correspondingly numbered physical extent blocks in each of the physical extents comprising a redundancy group.

The check data blocks in a redundancy group stripe protect the protected space in that stripe.

redundancy group stripe depth

[Storage System] The number of consecutively numbered physical extent blocks in one physical extent (strip) of a redundancy group stripe.

In the conventional striped data mapping model, redundancy group stripe depth is the same for all stripes in a redundancy group.

redundant (components)

[Computer System] Components of a system that have the capability to substitute for each other when one of the components fails, so that the system can continue to perform its function.

In storage subsystems, power distribution units, power supplies, cooling devices, and controllers are often configured to be redundant. The disks comprising a mirror set are redundant. A parity RAID array’s member disks are redundant, since surviving disks can collectively replace the function of a failed disk.

redundant (configuration, system)

[Computer System] A system or configuration of a system in which failure tolerance is achieved by the presence of redundant instances of all components that are critical to the system’s operation.

Redundant Array of Independent Disks (RAID)

[Storage System] A disk array in which part of the physical storage capacity is used to store redundant information about user data stored on the remainder of the storage capacity.

The redundant information enables regeneration of user data in the event that one of the array's member disks or the access path to it fails. Although it does not conform to this definition, disk striping is often referred to as RAID (RAID Level 0). See Berkeley RAID levels.

Redundant Array of Independent Nodes

[Storage] A collection of networked server nodes with software that provides and maintains pools of highly available storage capacity.

reference data

[Data Management] Synonym for fixed content.

reference information

[Data Management] Synonym for fixed content.


1. [General] Synonym for beer.

2. [Long Term Retention] A type of migration where the contents of some media are copied onto newer media of the same type.


[Storage System] Recreation of user data from a failed disk in a RAID array using check data and user data from surviving members.

Regeneration may also be used to recover data from an unrecoverable media error. Data in a parity RAID array is regenerated by computing the exclusive OR of the contents of corresponding blocks from the array's remaining disks. Data in a RAID Level 6 array is regenerated by choosing the more convenient of two parity algorithms and executing it.

registered state change notification

[Fibre Channel] A Fibre Channel switch function that allows notification to registered nodes if a change occurs to other specified nodes.

rejoin mirror

[Storage System] To bring a split mirror component back into the mirror.

When the resilvering process completes, the mirror component becomes identical to all mirror components in the mirror.


[Data Security] The process of changing the key used for an ongoing communication session.

relative offset

[Fibre Channel] A displacement, expressed in bytes, used to divide a quantity of data into blocks and subblocks for transmission in separate frames.

Relative offsets are used to reassemble data at the receiver and verify that all data has arrived.

relative offset space

[Fibre Channel] A numerical range defined by a sending upper level protocol for an information category.

The range starts at zero, representing the upper level-defined-origin, and extends to a highest value. Relative offset values are required to lie within the appropriate relative offset space.

Remote Authentication Dial In User Service (RADIUS)

[Data Security] An authentication and accounting protocol used by many Internet Service Providers (ISPs).

Information such as username and password is entered when a connection is made. This information is passed to a RADIUS server that verifies the information in order to authorize access to the system. RADIUS is defined in RFC 2865.

removable media library

[Backup] Synonym for library.

removable media storage device

[Storage System] A storage device designed so that its storage volumes can be readily removed and inserted.

Tapes, CD-ROMs, and optical disks are removable media devices.

replacement disk

[Storage System] A disk available for use as or used to replace a failed member disk in a RAID array.

Replacement Unit (RU)

[General] A component or collection of components in a system that are always replaced (swapped) as a unit when any part of the collection fails.

Replacement units may be field replaceable, or they may require that the system of which they are part be returned to a factory or repair depot for replacement. Field replaceable units may be customer replaceable, or their replacement may require trained service personnel. Typical replacement units in a disk subsystem include disks, controller logic boards, power supplies, cooling devices, and cables. Replacement units may be cold swappable, warm swappable, or hot swappable.

replay attack

[Data Security] An attack in which a valid data transmission is maliciously or fraudulently repeated, either by the originator or by an adversary who intercepts the data and retransmits it.


1. [Data Recovery] A general term for a copy of a collection of data. See duplicate, point in time copy, snapshot.

2. [Data Recovery] An image of data usable by one or more applications without an intermediate restore process.

See backup copy.


1. [Data Recovery] A general term for a copy of a collection of data. See duplicate, point in time copy, snapshot.

2. [Data Recovery] The action of making a replicate as defined above.

replication link

[Storage System] A physical and logical connection that transports data and replication control commands between primary and secondary sites.

replication set

[Storage System] A pair of volumes that have a replication relationship.

A replication set consists of a primary volume and a secondary volume that are physically separated. The replication set also defines how the primary and secondary volumes are connected and how replication ought to proceed.

Representational State Transfer (REST)

[Services] A specific set of principles for defining, addressing and interacting with resources addressable by URIs.

Architectures that follow these principles are said to be RESTful. The principles include: abstraction of state into resources and a uniform set of representations and operations (e.g., HTTP verbs like GET and PUT as the only means to manipulate a resource). RESTful interfaces are contrasted with Web Services interfaces such as WBEM, which tend to be RPC-like.


[Data Security] The act of a principal in denying, disowning or disavowing an act, event or transaction.

Request For Comment (RFC)

[Data Security] Internet -related specifications, including standards, experimental definitions, informational documents and best practice definitions, produced by the IETF.

request intensive (application)

[Computer System] A characterization of I/O intensive applications.

Request-intensive applications' I/O requests are usually randomly addressed and often specify a small amount of data for transfer.

reserved (field)

1. [General] In a standard, a field in a data structure set aside for future definition.

Some standards prescribe implementation behavior with respect to reserved fields (e.g., originators of data structures containing reserved fields must zero fill them; consumers of data structures containing reserved fields must ignore them, etc.); others do not.

2. [Fibre Channel] A field filled with binary zeros by a source N_Port and ignored by a destination N_Port.

Each bit in a reserved field is denoted by "r" in the Fibre Channel standards. Future enhancements to Fibre Channel Standards may define usages for reserved fields. Implementations should not check or interpret reserved fields. Violation of this guideline may result in loss of compatibility with future implementations that comply with future enhancements to Fibre Channel Standards.


[Storage] The ability of a storage element to preserve data integrity and availability of access despite the unavailability of one or more of its storage devices.


[Data Recovery] [Storage Systems] Synonym for mirror resynchronization.

resource pooling

[Computer System] Aggregation of a provider’s computing resources to serve multiple consumers using a multitenant model, with physical and virtual resources dynamically assigned and reassigned on demand.


1. [General] [iSCSI] In a negotiation or exchange, the party that responds to the originator of the negotiation or exchange.

2. [Fibre Channel] With reference to an Exchange, the Nx_Port that receives the frame that caused the Exchange to become open.

Responder Exchange Identifier (RX_ID)

[Fibre Channel] An identifier assigned by a responder to identify an exchange.

An RX_ID is meaningful only to the responder that originates it.


[Services] Abbreviation for Representational State Transfer.


[Data Recovery] Synonym for recovery.

retention period

1. [Data Recovery] The length of time that a backup image should be kept.

2. [File System] In some file systems, such as that shipped with IBM Corporation's OS/390 operating system, a property of a file that can be used to implement backup and data migration policies.

3. [Data Management] The length of time a compliance volume or file must be maintained undeleted and unchanged.

retention policy

[Data Management] A policy governing when and for how long a record must be retained by a storage system.

This may be a rule that applies to groups or categories of records, or may be specified for individual records. The policy may be time or event based.


[Computer System] A circuit that uses a clock independent of the incoming signal to generate an outbound signal.

return loss

[Data Communication] The ratio of the strength of a returned signal to that of the incident signal that caused it.

In electrical circuits, return loss is caused by impedance discontinuities. Optical return loss is caused by index of refraction differences.

reverse rejoin mirror

[Storage System] An operation performed on a split mirror component, in order to use its data as the basis for the mirror as a whole.

When a component is reverse rejoined, all mirror components in the mirror become identical to the previously split mirror component as a result of the resilver process.


[Data Security] Acronym for Request For Comment.


[Data Security] The potential that a given threat will exploit vulnerabilities of an asset or group of assets to cause loss or damage to the assets [IEEE 13335-1:1996].

risk acceptance

[Data Security] Decision to accept a risk.

[ISO/IEC 27000]

risk analysis

[Data Security] The process of identifying security risks, determining their magnitude, and identifying areas needing safeguards [IEEE 13335-1:1996].

risk management

[Data Security] The process of assessing and quantifying risk and establishing an acceptable level of risk for the organization [IEEE 13335-1:1996].

risk treatment

[Data Security] Process of selection and implementation of measures to modify risk.

[ISO/IEC 27000]

robotic media handler

[Storage System] The mechanical component of a library which moves removable volumes among drives and entry/exit slots.

Role-based Access Control (RBAC)

[Data Security] An access control method that assigns permissions to roles that mirror the organization and policies of an organization.

rollback to snapshot

1. [Storage System] The process of resetting a volume’s data to become identical to a snapshot taken of that volume.

2. [File System] In LTFS, the process of modifying the index to match a previous version of the index.

rotational latency

[Storage System] The interval between the end of a disk seek and the time at which the starting block address specified in the I/O request passes the disk head.

Exact rotational latencies for specific sequences of I/O operations can only be obtained by detailed disk drive simulation or measurement. The simplifying assumption that on average, requests wait for half a disk revolution time of rotational latency works well in practice. Half of a disk revolution time is therefore defined to be the average rotational latency.

routing function

[Fibre Channel] An entity that resides in an Inter-Fabric Router and is responsible for the forwarding of frames between independent Fabrics including the translation of N_Port_ID’s to present the Proxy N_Ports to the local Fabrics.


[Storage System] The set of blocks with corresponding physical extent block addresses in each of an array’s member physical extents.

The concept of rows is useful for locking the minimal amount of data during a RAID array update so as to maximize the potential for parallel execution.


[Data Recovery] Acronym for Recovery Point Objective.


[Data Security] Acronym for both a public key algorithm and a corporation in the business of algorithm design, derived from the names of its founders (Rivest, Shamir & Adelman).


[Fibre Channel] Acronym for Registered State Change Notification.


[Data Recovery] Acronym for Recovery Time Objective.


[General] Acronym for Replaceable Unit.


run length

[Data Communication] The number of consecutive identical bits in a transmitted signal. For example, the pattern 0011111010 has run lengths of 2, 5, 1, 1, and 1.

running disparity

[Fibre Channel] In a data stream using 8B/10B encoding, the cumulative disparity (positive or negative) of all previously issued transmission characters.


[Data Recovery] Acronym for Recorded Volume Serial Number.