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RADIUS

CONTEXT [Security]

Acronym for Remote Authentication Dial In User Service.

RAID

CONTEXT [Storage System]

1. An Acronym for Redundant Array of Independent Disks, a family of techniques for managing multiple disks to deliver desirable cost, data availability, and performance characteristics to host environments.
2. A Redundant Array of Independent Disks.
3. A phrase adopted from the 1988 SIGMOD paper A Case for Redundant Arrays of Inexpensive Disks.

RAID 0

RAID Level 0

CONTEXT [Storage System]

Synonym for data striping.

RAID 1

RAID Level 1

CONTEXT [Storage System]

Synonym for mirroring.

RAID 2

RAID Level 2

CONTEXT [Storage System]

A form of RAID in which a Hamming 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 3

RAID Level 3

CONTEXT [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 4

RAID Level 4

CONTEXT [Storage System]

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

RAID 5

RAID Level 5

CONTEXT [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 6

RAID Level 6

CONTEXT [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.

RAID array

CONTEXT [Storage System]

Acronym for Redundant Array of Independent Disks.

RAMdisk

CONTEXT [Storage Device

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. cf. solid state disk

random I/O

random I/O load

random reads

random writes

CONTEXT [Storage System]

Any I/O load whose consecutively issued read and/or write requests do not specify adjacently located 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. cf. sequential I/O

random number

CONTEXT [Security]

An unpredictable number used for cryptographic applications that is typically a generated sequence of zero and one bits. There are two basic classes: deterministic or pseudorandom often generated by an algorithm that produces a sequence of bits from an initial value called a seed; and nondeterministic generated by some unpredictable physical source that is outside human control.

random relative offset

CONTEXT [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.

rank

CONTEXT [Storage System]

1. A set of physical disk positions in an enclosure, usually denoting the disks that are or can be members of a single array.
2. The set of corresponding target identifiers on all of a controller’s device I/O buses. Like the preceding definition, the disks identified as a rank by this definition usually are or can be members of a single array
3. 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.

RAS

1. Acronym for Reliability, Availability, and Serviceability
2. Acronym for Remote Access Server (Windows NT dialup networking server).

raw partition

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

CONTEXT [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. cf. disk image backup

read/write head

CONTEXT [Storage Device

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.

real time copy

CONTEXT [Storage System]

Synonym for mirroring.

rebuild

rebuilding

CONTEXT [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.

receiver 

CONTEXT [General] [Fibre Channel]

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.

receptacle

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

reconstruction

CONTEXT [Storage System]

Synonym for rebuilding.

recorded volume serial number

CONTEXT [Data Recovery]

Synonym for media ID. Abbreviated RVSN.

recovery

CONTEXT [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. cf. restoration

Recovery Point Objective - RPO

CONTEXT [Data Recovery]

The maximum desired 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

CONTEXT [Data Recovery]

The maximum desired time period required to bring one or more applications and associated data back to a correct operational state.

red

CONTEXT [Security]

In the context of security analysis, a designation applied to information systems and associated areas, circuits, components, and equipment in which sensitive information is being processed.

red/black concept

CONTEXT [Security]

The separation of electrical and electronic circuits, components, equipment and systems that handle sensitive information (red) in electrical form, from those that handle on information that is not sensitive (black) in the same form.

reduced mode

CONTEXT [Storage System]

Synonym for degraded mode.

reduction

CONTEXT [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.

redundancy

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

CONTEXT [Management] [Storage System]

1. A collection of extents organized by 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
2. A class defined in the CIM schema (CIM_RedundancyGroup) consisting of a collection of objects in which redundancy is provided. Three subclasses of CIM_RedundancyGroup are defined (1.) CIM_SpareGroup for sparing and failover, (2.) CIM_ExtraCapacityGroup for load sharing or load balancing, and (3.) CIM_StorageRedundancyGroup to describe the redundancy algorithm in use.

redundancy group stripe

CONTEXT [Storage System]

A set of sequences 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

CONTEXT [Storage System]

The number of consecutively numbered physical extent blocks in one physical extent 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)

Components of a system that have the capability to substitute for each other when necessary, as, for example 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)

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

CONTEXT [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).

regeneration

CONTEXT [Information Lifecycle Management]

Synonym for fixed content.

regeneration

CONTEXT [Information Lifecycle Management]

Synonym for fixed content.

regeneration

CONTEXT [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

CONTEXT [Fibre Channel]

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

rejoin mirror

CONTEXT [Storage System]

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.

rekeying

CONTEXT [Security]

The process of changing the key used for an ongoing communication session.

relative offset 

CONTEXT [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

CONTEXT [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

CONTEXT [Security]

RADIUS is 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 authorizes access to the system. Radius is defined in RFC 2865.

removable media storage device 

A storage device designed so that its storage media can be readily removed and inserted. Tapes, CDROMs, and optical disks are removable media devices.

replacement disk

CONTEXT [Storage System]

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

replacement unit

A component or collection of components in a system which are always replaced (swapped) as a unit when any part of the collection fails. Abbreviated RU. 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, warm, or hot swappable.

replay attack

CONTEXT [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.

replica

CONTEXT [Data Recovery]

1. A general term for a copy of a collection of data. cf. duplicate, point in time copy, snapshot
2. An image of data usable by one or more applications without an intermediate restore process. cf. backup copy.

replicate

CONTEXT [Data Recovery]

1. (noun) A general term for a copy of a collection of data. cf. duplicate, point in time copy, snapshot
2. (verb) The action of making a replicate as defined above.

replication link

CONTEXT [Storage System]

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

replication set

CONTEXT [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.

Request for Comment

CONTEXT [Security]

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

request intensive (application)

A characterization of applications. A request-intensive application is an I/O intensive application characterized by a high rate of I/O requests. Request-intensive applications' I/O requests are usually randomly addressed and often specify a small amount of data for transfer.

reserved (field)

CONTEXT [General] [Fibre Channel]

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 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 which comply with future enhancements to Fibre Channel Standards.

resilvering

CONTEXT [Data Recovery, Storage Systems]

Synonym for mirror resynchronization.

responder 

CONTEXT [General][iSCSI] [Fibre Channel]

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

CONTEXT [Fibre Channel]

An identifier assigned by a responder to identify an exchange. Abbreviated RX_ID. An RX_ID is meaningful only to the responder that originates it.

restoration

CONTEXT [Data Recovery]

The copying of a backup to on-line storage for application use. Restoration normally occurs after part or all of an application’s data has been destroyed or become inaccessible. cf. recovery

retention period

CONTEXT [Data Recovery] [File System]

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. Compliance] The length of time a compliance volume or file must be maintained undeleted and unchanged.

Retention policy

CONTEXT [Information Lifecycle 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.

retimer

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

return loss

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.

RFC

CONTEXT [Security]

Acronym for Request for Comment

robot

robotic media handler

CONTEXT [Data Recovery]

A mechanical handler capable of storing multiple pieces of removable media and loading and unloading them from one or more drives in arbitrary order in response to electronic commands. cf. library

risk

CONTEXT [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 analysis

CONTEXT [Security]

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

risk management

CONTEXT [Security]

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

rollback to snapshot

CONTEXT [Storage System]

The process of resetting a volume's data to become identical to a snapshot taken of that volume.

rotational latency

CONTEXT [Storage Device]

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

CONTEXT [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.

row

CONTEXT [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.

RSA

CONTEXT [Security]

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

RSCN

CONTEXT [Fibre Channel]

Acronym for registered state change notification.

RU

Acronym for replaceable unit. cf. CRU, FRU

run length

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

CONTEXT [Fibre Channel]

The cumulative disparity (positive or negative) of all previously issued transmission characters.

RVSN 

CONTEXT [Data Recovery]

Acronym for recorded volume serial number.

Key Programs

• Data Management Forum
• Green Storage Initiative
• Storage Management Initiative (SMI)
• XAM Initiative