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Interface spec for IDE drives
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The process whereby the initialization process of a disk array takes place in the background. Use of background initialization allows use of a disk array within minutes instead of several hours. Also known as Immediate RAID Availability.
A block (usually the size of a sector) that cannot reliably hold data due to a physical flaw of damaged format markings.
Transmission capacity of a communication line or bus, expressed in Hertz
Stands for "binary digit", the smallest unit of data, represented by a series of "0" and/or "1"
An amount of data moved as a single unit; smallest amount of data that can be read or written at a time.
Allocated memory space used for temporary storage
Unit of digital data comprising eight bits, sometimes referred to as a "character"
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A fast, random-access system for storing frequently accessed data in RAM
The amount of information, expressed in bytes, that can be stored on a hard drive. Also known as storage capacity.
General term for path on which electronic signals travel
A cluster is a group of terminals or workstations attached to a common control unit or server or a group of several servers that share work and may be able to back each other up if one server fails.
System must be shut down and power turned off before replacing drive in an array
Aggregate of all tracks that reside in the same location on every disk surface
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Temporary storage of new write data or high-demand read data in solid state memory in order to accelerate performance. The cached data is later overwritten with newly cached data once it is either written to disk or deemed to be of low demand.
The rate at which data is transferred to or from a storage device, expressed in KB or MB per second.
Combining more commonly accessible disk drives for more capacity, speed, or for disaster recovery.
A device for storing and retrieving data on a disk
A fault-tolerant technique that writes data simultaneously to two or more disks using the same disk controller.
Spreading data over multiple disks drives
Direct Memory Access.
A process for transferring data directly to and from main memory, without passing through the CPU. DMA improves speed and efficiency by allowing the system to continue processing even while it is retrieving new data from the drive.
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Error Correction Code.
The incorporation of extra parity bits in transmitted data so that the controller can correct detected errors.
Hard disk interface that provides data transfer up to 13 MBps
A process based on a mathematical algorithm that is used by RAID levels 3 or 5 to compare data created by a read request during a drive failure or by write request.
Storage of data outside of the CPU, such as disk or tape
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A mode of operation for failure tolerant systems in which a component has failed and its function has been assumed by a redundant component
Redundant components in storage system provide security against system failure
File Allocation Table maintained by an operating system on a hard disk that provides a map of the clusters (the basic unit of logical storage on a hard disk) that a file has been stored in. FAT 32 supports 32-bit processors.
A technology for transmitting data between computer devices at a data rate of up to 2 Gbps (Two billion bits per second), especially suited for connecting computer servers to shared storage devices and for interconnecting storage controllers and drives. It is also more flexible; devices can be as far as ten kilometers (about six miles) apart. The longer distance requires optical fiber as the physical medium.
Permanent instructions and data programmed directly into a circuitry of read-only memory for controlling the operation of the devices. Distinct from software, which can be altered by programmers.
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Graphical User Interface
A software interface that interacts with the user through color graphics and a mouse
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Any computer system to which disks are attached and accessible for data storage and I/O
Host Bus Adapter
An I/O adapter that connects a host I/O bus to the host's storage memory system
The operation of adding or removing a device from a bus while transactions involving other devices are occurring over the bus
RAID storage feature that allows a spare drive to be configured on-line for automatic reconstruction in the event of disk failure.
A redundant component is a fault tolerant storage system that has power applied and is ready to operate, but which does not perform its task as long as the primary component for which it is standing by is functioning properly.
Replacing one member disk in a RAID while the RAID is still operating
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Just A Bunch of Disks (Drives), a number of disk drives, usually in an enclosure JBOD implies that the disks are not RAIDed and function independently.
Sometimes referred to as "Just a Bunch of Drives." Each drive is operated independently like a normal disk controller, or drives may be spanned and seen as a single drive. This level does not provide data redundancy.
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Kilobytes
A unit of measure consisting of 1,024 bytes.
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System Drives resented to the operating system as available disk drives, each with a capacity.
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Megabyte
A megabyte is 106 or 1,000,000 bytes. One megabyte can store more than one million characters.
Also known as RAID 1. Full redundancy is obtained by duplicating all data from primary disk on rest disks.
Mean time between failures. The average time a device works without failure.
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An extra bit, added to a number, used for checking the accuracy of binary members.
The special set of rules for communicating that the end points in a telecommunication connection use when they send signals back and forth.
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Redundant Arrays of Independent/Inexpensive Disks. Two or more drives working together that provides increased performance and various levels of error recovery and fault tolerance.
RAID (Redundant Array of Independent Disks) is an acronym first used in a 1988 paper by Berkeley researchers Patterson, Gibson and Katz. It described array configuration and applications for multiple inexpensive hard disks, providing fault tolerance (redundancy) and improved access rates.
RAID provides a method of accessing multiple individual disks as if the array were one large disk, spreading data access out over these multiple disks, thereby reducing the risk of losing all data if one drive fails, and improving access time.
Block "striping" across multiple drives is provided, yielding higher performance than is possible with individual drives. This level does not provide any redundancy.
Also known as "Disk Striping", this is technically not a RAID level since it provides no fault tolerance. Data is written in blocks across multiple drives, so one drive can be writing or reading a block while the next is seeking the next block.
The advantages of striping are the higher access rate, and full utilization of the array capacity. The disadvantage is there is no fault tolerance-if one drive fails, the entire contents of the array become inaccessible.
Drives are paired and mirrored. All data is 100 percent duplicated on a drive of equivalent size.
Known as "Disk Mirroring" provides redundancy by fully duplicating drive data to all other drives in the array. If one drive fails, the others contain exact duplicate of the data and the RAID can switch to using the mirror drive with no lapse in user accessibility.
The disadvantages of mirroring are no improvement in data access speed, and capacity is low. However, it provides the best protection of data since the array management software will simply direct all application requests to the surviving disk members when a member of disk fails
Data is "striped" across several physical drives. Maintains parity information which can be used for data recovery.
RAID level 3 stripes data across multiple drives, with an additional drive dedicated to parity, for error correction & recovery.
Data is "striped" across several physical drives. For data redundancy, drives are encoded with rotated XOR redundancy.
RAID level 5 is the most popular configuration, providing striping as well as parity for error recovery. In RAID 5, the parity block is distributed among the drives of array, giving a more balanced access load across the drives. The parity information is used to recover data if one drive fails, and this method is the most popular. The disadvantage is a relatively slow write cycle (2 reads and 2 writes are required for each block written). The array capacity is N-1, with a minimum of 3 drives required.
RAID 6 improves over the industry standard RAID 5 by adding another parity disk which gives greater reliability and data protection.
Combines RAID 0 striping and RAID 1 mirroring. This level provides redundancy through mirroring.
Combines RAID 0 striping and RAID 1 mirroring spanned across multiple drive groups (super drive group). This level provides redundancy through mirroring.
Data is "striped" across multiple drive groups (super drive group). Maintains parity information which can be used for data recovery.
Data is "striped" across multiple drive groups (super drive group). For data redundancy, drives are encoded with XOR redundancy.
A feature in RAID subsystems that allows for changing a RAID level to another level without powering down the system.
The process of reconstructing data from a failed disk using data from other drives.
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SED (Self-Encrypting Drives). Encryption controller (ASIC) & Encryption Key are both embedded on hard drive itself. SED encryption is automatic and transparent without performance degradation. Write-Data is encrypted with an encryption key as the data is written on the disk. Read-Data is decrypted with the encryption key as the data is retrieved from the disk.
Small Computer System Interface. A hardware interface that allows for the connection several peripheral devices. Narrow SCSI supports up to 7 devices on a channel, and Wide SCSI, Ultra SCSI, and Ultra2/Ultra3/Ultra 320 SCSI support up to 15 devices on a channel.
Data communications mode in which bits are sent in sequence; contrast with parallel interface.
Self Monitoring Analysis and Reporting Technology, the industry standard reliability prediction indicator for both the IDE/ATA and SCSI hard disk drives. Hard disk drives with SMART offer early warning of some hard disk failures so critical data can be protected.
A high-speed, open-standard scalable network of storage devices and servers providing accelerated data access.
Spreading data evenly over multiple disk drives to enhance performance.
The stripe size is defined as the size, in kilobytes (1024 bytes) of a single I/O operation. A stripe of data (data residing in actual physical disk sectors, which are logically ordered first to last) is divided over all disks in the drive group.
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A feature in SCSI that enables disk drives and I/O adapters to send multiple commands prior to completion of a given command.
The number of I/O requests satisfied per unit time (usually per second.).
The rate at which bytes or bits are transferred, usually expressed as MBps.
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Ability to remove and replace a disk drive while power is on.
Typically the RAID is used in large file servers, transaction of application servers, where data accessibility is critical, and fault tolerance is required. Nowadays, RAID is also being used in desktop systems for CAD, multimedia editing and playback where higher transfer rates are needed.
Often referred to as WORM (write-once, read-many) drives, data recorded on these devices cannot be erased or altered.
Refers to a caching strategy whereby data is written to the drives before a completion status is returned to the host operating system. This caching strategy is considered more secure, since a power failure will be less likely to cause loss of data. However, a write through cache results in a slightly lower performance.
Refers to a caching strategy whereby write operations result in a completion signal being sent to the host operating system as soon as the cache (not the disk drive) receives the data to be written. The target drive will receive the data at a more appropriate time, in order to increase controller performance.
